assertion on runtime causes program to stop when accessing const json with missing key

[codeaddicted]

Hi,

i just stumbled upon something i think might be a bug but am unsure about.

when accessing a const nlohmann::json &j object and using a missing key the library will fail with an assertion which causes the whole program to stop (assertion not catchable).

Here is an example how to reproduce it.

#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main() {
  try {
    const json j{
        {"foobar", "barfoo"},
        {"boofar", "blubber"}
    };

    std::cout << j.dump(2) << std::endl; // check that it
    std::cout << j["fubber"] << std::endl;
  } catch ( json::exception &e ) {
    std::cout << "JSON Exception: " << e.what() << std::endl;
  } catch (std::exception &e ) {
    std::cout << "Std Exception: " << e.what() << std::endl;
  }

  std::cout << "Successfully Finished!" << std::endl;
}

i know i should check for a keys existence with j.count(<key>) but shouldn't the access on a missing key throw an exception of type json::out_of_range (or similar) so it is catchable instead of asserting and therefore aborting the whole program?

The error it produces is:

nlohmann_assert_error_test: /tmp/tmp.HiflujZ3cJ/./nlohmann-json/include/nlohmann/json.hpp:3735: const value_type& nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::operator[](T*) const [with T = const char; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::const_reference = const nlohmann::basic_json<>&; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::value_type = nlohmann::basic_json<>]: Assertion `m_value.object->find(key) != m_value.object->end()' failed.

Which compiler and operating system are you using?

• Compiler: g++ (Debian 8.3.0-6) 8.3.0
• Operating system: Debian 10.4

Which version of the library did you use?

current develop (@1bcabd9e83ebc2d04b5767c7f51bf385e80398d6)

• latest release version 3.9.1
• other release - please state the version: ___
• the develop branch

[t-b]

Only at does bounds checking, see https://nlohmann.github.io/json/api/basic_json/at/. The index operator does no bounds checking.

[codeaddicted]

Well is there a reason why or just "because"?

[nlohmann]

This is not a bug, but a feature. Unchecked access is apparently faster than bounded-checked access, and accessing a nonexisting element of a const object is undefined behavior. This is described in https://nlohmann.github.io/json/api/basic_json/operator%5B%5D/. A more detailed description can be found in https://nlohmann.github.io/json/features/element_access/unchecked_access/ . Please let me know if further information is needed.

[gregmarr]

See #2111 for discussion of a possible solution to this.

[codeaddicted]

Thank you for clarifying this. I know understand why the function behaves like this. Closing this issue.