try/catch block doesn't work while accessing const json& array.

[itamarcps]

Description

try/catch block with a function taking const json& as a argument and accessing an array within the json doesn't catch.

Reproduction steps

Reproducible with the code included in this issue.

Expected vs. actual results

The function should be able to catch, if you take out the const from the argument, it catches successfully.

Minimal code example

#include <iostream>
#include "json.hpp"

using json = nlohmann::ordered_json;

void function(const json& request) {
  try {
    std::cout << "Trying to get" << std::endl;
    std::cout << request.dump() << std::endl;
    const auto value = request["information"][0].get<std::string>();
    std::cout << "Got" << std::endl;
  } catch(std::exception &e) {
    std::cout << "Caught Exception" << std::endl;
    throw std::runtime_error("Error");
  }
}

int main() {
  json request;
  request["information"] = json::array();
  try {
    function(request);
  } catch (std::exception &e) {
    std::cout << "Catched!" << std::endl;
    std::cout << e.what() << std::endl;
  }
 return 0;
}

Error messages

The function segfaults instead of throwing.

Compiler and operating system

gcc 10.2.1

Library version

C++20, Latest nlohmann/json release.

Validation

• [X] The bug also occurs if the latest version from the develop branch is used.
• [X] I can successfully compile and run the unit tests.

[gregmarr]

It is undefined behavior to use operator[] on a const object for an array index that doesn't exist. You need to be sure that it exists first. If you want checked access, you need to use at() instead. https://json.nlohmann.me/api/basic_json/operator%5B%5D/#notes

[itamarcps]

Thanks, using .at(idx) works perfectly fine for me.