Is it possible to have template json::parse with noexcept specifier?

[deiuch]

• Describe what you want to achieve.
  Have noexcept for json::parse methods if the third argument is false.

• Describe what you tried.
  I've tried to use this method in my exception-free code, but my IDE (especially ReSharper C++ plugin) warned me about unhandled exceptions from calling this function. Also noexcept specifier makes optimization passes to be more aggressive.

• Describe which system (OS, compiler) you are using.
  Microsoft Windows 10, MSVC (from Microsoft Visual Studio 2017), CMake (from 3.12), Vcpkg (version 2020.01.17-nohash).

• Describe which version of the library you are using (release version, develop branch).
  Version available as the last in vcpkg (in library source marked as 3.7.3).

[deiuch]

Somehow this was not inserted into issue text, I'll paste it here.

json/single_include/nlohmann/json.hpp

Lines 20744 to 20747 in 6a6790a

	JSON_HEDLEY_WARN_UNUSED_RESULT
	static basic_json parse(detail::input_adapter&& i,
	const parser_callback_t cb = nullptr,
	const bool allow_exceptions = true)

[deiuch]

I've found a macro that may affect exception policy of the library.
Is it possible to introduce some macro like JSON_NOEXCEPT that will put the corresponding noexcept declaration to every function like json::parse based on current JSON_NOEXCEPTION macro?

json/single_include/nlohmann/json.hpp

Lines 1747 to 1758 in 6a6790a

	#if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
	#define JSON_THROW(exception) throw exception
	#define JSON_TRY try
	#define JSON_CATCH(exception) catch(exception)
	#define JSON_INTERNAL_CATCH(exception) catch(exception)
	#else
	#include <cstdlib>
	#define JSON_THROW(exception) std::abort()
	#define JSON_TRY if(true)
	#define JSON_CATCH(exception) if(false)
	#define JSON_INTERNAL_CATCH(exception) if(false)
	#endif

Example:

#if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
    /* ... */
    #define JSON_NOEXCEPT noexcept(false)
#else
    /* ... */
    #define JSON_NOEXCEPT noexcept
#endif

/* ... */

    JSON_HEDLEY_WARN_UNUSED_RESULT
    static basic_json parse(detail::input_adapter&& i,
                            const parser_callback_t cb = nullptr,
                            const bool allow_exceptions = true)
    JSON_NOEXCEPT
    {
        /* ... */
    }

/* ... */

// clean up
/* ... */
#undef JSON_NOEXCEPT

[nlohmann]

Technically, the parse function is not noexcept, because it allocates memory which can yield std::bad_alloc.

[deiuch]

Got it, so no more questions :)
But you still may consider this macro (if it has any sense for some function in public API).