std::basic_string requires Allocator to have a default constructor

[llvmbot]

Bugzilla Link	24933
Resolution	INVALID
Resolved on	Sep 24, 2015 23:48
Version	3.6
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@mclow

Extended Description

Instantiating std::basic_string<char, std::char_traits, MyAllocator> fails if MyAllocator doesn't have a default constructor. Allocators are not supposed to require a default constructor, and with stateful allocators in C++11 a default constructor doesn't make sense.

Tried with Xcode 5.1.1 (Apple clang 3.6) and a custom OSX clang 3.6 build. I assume this means libc++ 3.6; I don't know how to check the libc++ version.

test.cpp:
#include
#include

template
class MyAllocator {
public:
typedef T value_type;

// Uncomment to compile successfully. Should not be needed.
// MyAllocator() {}

template <typename U>
MyAllocator(const MyAllocator<U>&& other) {}

T* allocate(size_t n) {
    return static_cast<T*>(malloc(n * sizeof(T)));
}

void deallocate(T* p, size_t /*n*/) {
    free(p);
}

};

int main(int /argc/, char* /argv/[]) {
std::basic_string<char, std::char_traits, MyAllocator> str("Hello, world!");
return 0;
}

$ clang --version
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.5.0
Thread model: posix

$ clang -Wall -std=c++11 -c test.cpp -o test.o
In file included from test.cpp:2:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:439:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:628:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:2136:31: error: constructor for 'std::__1::__libcpp_compressed_pair_imp<std::__1::basic_string<char, std::__1::char_traits, MyAllocator >::__rep, MyAllocator, 2>' must explicitly initialize the base class 'MyAllocator' which does not have a default constructor
_LIBCPP_INLINE_VISIBILITY __libcpp_compressed_pair_imp() {}
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:2315:31: note: in instantiation of member function 'std::__1::__libcpp_compressed_pair_imp<std::__1::basic_string<char, std::__1::char_traits, MyAllocator >::__rep, MyAllocator, 2>::__libcpp_compressed_pair_imp' requested here
_LIBCPP_INLINE_VISIBILITY __compressed_pair() {}
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1387:31: note: in instantiation of member function 'std::__1::__compressed_pair<std::__1::basic_string<char, std::__1::char_traits, MyAllocator >::__rep, MyAllocator >::__compressed_pair' requested here
_LIBCPP_INLINE_VISIBILITY basic_string(const value_type* __s);
^
test.cpp:25:72: note: in instantiation of member function 'std::__1::basic_string<char, std::__1::char_traits, MyAllocator >::basic_string' requested here
std::basic_string<char, std::char_traits, MyAllocator> str("Hello, world!");
^
test.cpp:5:7: note: 'MyAllocator' declared here
class MyAllocator {
^
1 error generated.

[llvmbot]

assigned to @mclow

[mclow]

I don't think that you've interpreted the standard correctly.

The call that you're making has the following signature:
[string.cons], just before p8

basic_string(const charT* s, const Allocator& a = Allocator());

Note: Libc++ actually implements two different signatures:
basic_string(const charT s);
basic_string(const charT s, const Allocator& a);

and you're calling the first one.

In any case, the call needs an allocator to create the std::string object, and you have not provided one to the constructor. Hence the default construction.

I think that the appropriate entry in the standard is Table 98; the third line.

Consider the following:
std::basic_string<char, std::char_traits, MyAllocator> str;

which shows the same issue.

[llvmbot]

You're right. I'm terribly sorry. I was trying to simplify from something more complex, came up with the same error, and assumed I'd simplified correctly.

For what it's worth, the real problem is essentially:

typedef std::basic_string<char, std::char_traits, MyAllocator> MyString;
typedef std::unordered_map<MyString, void*, std::hash, std::equal_to, MyAllocator<std::pair<const MyString, void*>>> MyUnorderedMap;
...
MyUnorderedMap um;
um.insert(std::make_pair(str, ptr));

Something in insert() is default-constructing the pair<MyString,void*>, which default-constructs MyString, and fails.

I'll go do my homework properly this time and figure out if this is expected/correct, and if so, whether there's a valid way to avoid the default construction.