Support moving from rvalues in std::initializer_list #663

himikof · 2017-07-23T21:34:23Z

This should fix the underlying problem of inefficient list initialization, which has led me to filing #657.

By using a rvalue-aware proxy type inside all uses of std::initializer_list, simple, obvious usages such as

return json {
   {"type", "Polygon"},
   {"coordinates", std::move(coordinates)},
};

should just work without calling json copy constructor even once.

Possible downsides for this solution:

It can increase the number of move constructor calls in some situations. I think it is a good trade-off to make, because moving json values should be very cheap (and possibly optimized into nothing).
It is, strictly speaking, a breaking interface change, but it is only limited to somehow using an explicitly typed std::initializer_list values, which is not common in C++. All normal usage should remain unaffected (just a bit faster).
It is certainly a hack 😃

This commit works around an issue in std::initializer_list design. By using a detail::json_ref proxy with a mutable value inside, rvalue-ness of an input to list initializer is remembered and used later to move from the proxy instead of copying.

std::initializer_list does not own the temporaries created in its initialization. Therefore, storing it in an independent stack variable is unsafe.

C++ overload resolution/list initialization rules are hard.

coveralls · 2017-07-23T23:32:28Z

Coverage remained the same at 100.0% when pulling 0f4978e on himikof:move-construction into 4414f94 on nlohmann:develop.

coveralls · 2017-07-24T00:44:10Z

Coverage remained the same at 100.0% when pulling 0f4978e on himikof:move-construction into 4414f94 on nlohmann:develop.

coveralls · 2017-07-24T10:49:29Z

Coverage remained the same at 100.0% when pulling 897879b on himikof:move-construction into 4414f94 on nlohmann:develop.

On MSVC compiler, temporaries that are constructed during a list initialization, are sometimes destroyed even before calling the initializing constructor, instead of at the end of the containing full-expression. This is clearly non-conforming to [class.temporary]. As the impact of this bug is silently producing incorrect JSON values, move eagerly from rvalues to be safe. See https://stackoverflow.com/questions/24586411

coveralls · 2017-07-25T10:38:59Z

Coverage remained the same at 100.0% when pulling 93bb712 on himikof:move-construction into 4414f94 on nlohmann:develop.

nlohmann

Looks good to me.

nlohmann · 2017-07-30T18:00:52Z

Thanks a lot!

coveralls · 2017-07-30T18:22:46Z

Coverage remained the same at 100.0% when pulling aad5521 on himikof:move-construction into 96dd4ff on nlohmann:develop.

nlohmann · 2017-07-30T21:01:32Z

I have some conversion warnings when compiling with GCC with maximal warnings:

$  make pedantic_gcc
/Library/Developer/CommandLineTools/usr/bin/make json_unit CXX=g++ CXXFLAGS="\
		-std=c++11 \
		-Wno-deprecated-declarations \
		-Werror \
		-Wall -Wpedantic -Wextra \
		-Walloca \
		-Warray-bounds=2 \
		-Wcast-qual -Wcast-align \
		-Wchar-subscripts \
		-Wconditionally-supported \
		-Wconversion \
		-Wdate-time \
		-Wdeprecated \
		-Wdisabled-optimization \
		-Wdouble-promotion \
		-Wduplicated-branches \
		-Wduplicated-cond \
		-Weffc++ \
		-Wformat-overflow=2 \
		-Wformat-signedness \
		-Wformat-truncation=2 \
		-Wformat=2 \
		-Wno-ignored-qualifiers \
		-Wimplicit-fallthrough=5 \
		-Wlogical-op \
		-Wmissing-declarations \
		-Wmissing-format-attribute \
		-Wmissing-include-dirs \
		-Wnoexcept \
		-Wnonnull \
		-Wnull-dereference \
		-Wold-style-cast \
		-Woverloaded-virtual \
		-Wparentheses \
		-Wplacement-new=2 \
		-Wredundant-decls \
		-Wreorder \
		-Wrestrict \
		-Wshadow=global \
		-Wshift-overflow=2 \
		-Wsign-conversion \
		-Wsign-promo \
		-Wsized-deallocation \
		-Wstrict-overflow=5 \
		-Wsuggest-attribute=const \
		-Wsuggest-attribute=format \
		-Wsuggest-attribute=noreturn \
		-Wsuggest-attribute=pure \
		-Wsuggest-final-methods \
		-Wsuggest-final-types \
		-Wsuggest-override \
		-Wtrigraphs \
		-Wundef \
		-Wuninitialized -Wunknown-pragmas \
		-Wunused \
		-Wunused-const-variable=2 \
		-Wunused-macros \
		-Wunused-parameter \
		-Wuseless-cast \
		-Wvariadic-macros"
[CXX]   src/unit-constructor1.o
src/unit-constructor1.cpp: In function ‘void ____C_A_T_C_H____T_E_S_T____0()’:
src/unit-constructor1.cpp:205:129: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
             json j({{"a", json(1)}, {"b", json(1u)}, {"c", json(2.2)}, {"d", json(false)}, {"e", json("string")}, {"f", json()}});
                                                                                                                                 ^
src/unit-constructor1.cpp:205:129: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:205:129: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:205:129: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:205:129: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:205:129: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:205:129: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:205:129: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:348:87: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
             json j({json(1), json(1u), json(2.2), json(false), json("string"), json()});
                                                                                       ^
src/unit-constructor1.cpp:348:87: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:348:87: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:348:87: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
src/unit-constructor1.cpp:348:87: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:348:87: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:348:87: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
src/unit-constructor1.cpp:348:87: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:348:87: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:348:87: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
src/unit-constructor1.cpp:348:87: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:348:87: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:348:87: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
src/unit-constructor1.cpp:348:87: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:348:87: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:348:87: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
src/unit-constructor1.cpp:348:87: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:348:87: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:862:75: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
                     json j(json::initializer_list_t {json(json::array_t())});
                                                                           ^
src/unit-constructor1.cpp:862:75: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:862:75: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:877:76: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
                     json j(json::initializer_list_t {json(json::object_t())});
                                                                            ^
src/unit-constructor1.cpp:877:76: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:877:76: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:892:73: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
                     json j(json::initializer_list_t {json("Hello world")});
                                                                         ^
src/unit-constructor1.cpp:892:73: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:892:73: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:907:64: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
                     json j(json::initializer_list_t {json(true)});
                                                                ^
src/unit-constructor1.cpp:907:64: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:907:64: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:922:61: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
                     json j(json::initializer_list_t {json(1)});
                                                             ^
src/unit-constructor1.cpp:922:61: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:922:61: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:937:62: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
                     json j(json::initializer_list_t {json(1u)});
                                                              ^
src/unit-constructor1.cpp:937:62: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:937:62: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:952:65: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversio]
                     json j(json::initializer_list_t {json(42.23)});
                                                                 ^
src/unit-constructor1.cpp:952:65: error:   for conversion from ‘nlohmann::json {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:952:65: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:1123:50: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
                     json j {std::move(source), {}};
                                                  ^
src/unit-constructor1.cpp:1123:50: error:   for conversion from ‘std::remove_reference<nlohmann::basic_json<>&>::type {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:1123:50: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:1129:55: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
                     json j {{"key", std::move(source)}};
                                                       ^
src/unit-constructor1.cpp:1129:55: error:   for conversion from ‘std::remove_reference<nlohmann::basic_json<>&>::type {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:1129:55: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:1135:52: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
                     json j = {std::move(source), {}};
                                                    ^
src/unit-constructor1.cpp:1135:52: error:   for conversion from ‘std::remove_reference<nlohmann::basic_json<>&>::type {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:1135:52: note:   because conversion sequence for the argument is better
src/unit-constructor1.cpp:1141:57: error: choosing ‘nlohmann::detail::json_ref<BasicJsonType>::json_ref(nlohmann::detail::json_ref<BasicJsonType>::value_type&&) [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]’ over ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = nlohmann::detail::json_ref<nlohmann::basic_json<> >; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’ [-Werror=conversion]
                     json j = {{"key", std::move(source)}};
                                                         ^
src/unit-constructor1.cpp:1141:57: error:   for conversion from ‘std::remove_reference<nlohmann::basic_json<>&>::type {aka nlohmann::basic_json<>}’ to ‘nlohmann::detail::json_ref<nlohmann::basic_json<> >’ [-Werror=conversion]
src/unit-constructor1.cpp:1141:57: note:   because conversion sequence for the argument is better
cc1plus: all warnings being treated as errors
make[2]: *** [src/unit-constructor1.o] Error 1
make[1]: *** [json_unit] Error 2
make: *** [pedantic_gcc] Error 2

Any ideas on this?

nlohmann · 2017-07-30T21:08:52Z

Furthermore, there is a Weffc++ warning that json_ref has pointer members, but no copy constructor or assignment operator.

himikof · 2017-07-30T21:45:11Z

As for the conversion warnings, it seems to be an unwanted (but harmless) interplay with basic_json::operator ValueType() const, which is not supposed to be used with json_ref. So, SFINAE-blacklisting ValueType == json_ref there should get rid of this warning.

As for constructors, json_ref should only be movable, and default move constructor is fine on a conforming C++11 compiler. So, adding json_ref(json_ref &&) = default; should be enough.

#663 (comment)

himikof added 5 commits July 23, 2017 23:47

Add some tests for std::move from std::initializer_list

7b3cbff

Optimize json construction from rvalue string_t/array_t/object_t

cf3ca3b

Update tests while fixing possible UB

f5cae64

std::initializer_list does not own the temporaries created in its initialization. Therefore, storing it in an independent stack variable is unsafe.

Fix an actually invalid test

0f4978e

C++ overload resolution/list initialization rules are hard.

Make detail::json_ref do less work by deferring moves/copies

897879b

nlohmann approved these changes Jul 30, 2017

View reviewed changes

nlohmann self-assigned this Jul 30, 2017

nlohmann added the kind: enhancement/improvement label Jul 30, 2017

nlohmann added this to the Release 3.0.0 milestone Jul 30, 2017

Merge branch 'develop' into move-construction

aad5521

nlohmann merged commit 80e9584 into nlohmann:develop Jul 30, 2017

nlohmann added a commit that referenced this pull request Jul 30, 2017

🔨 cleanup after #663

850d856

himikof mentioned this pull request Jul 30, 2017

Adding additional push_back/operator+= rvalue overloads for JSON object #657

Closed

himikof deleted the move-construction branch July 30, 2017 21:58

nlohmann added a commit that referenced this pull request Jul 31, 2017

🔨 fixed GCC warnings in #663

e60002a

#663 (comment)

nlohmann mentioned this pull request May 17, 2018

Q: few questions about json construction #1092

Closed

nlohmann mentioned this pull request Oct 5, 2019

What is this json_ref? #1772

Closed

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Support moving from rvalues in std::initializer_list #663

Support moving from rvalues in std::initializer_list #663

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Support moving from rvalues in std::initializer_list #663

Support moving from rvalues in std::initializer_list #663

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