Passing an iteration object by reference to a function

Bug Report

• What is the issue you have?
  I cannot pass a JSON iteration object by reference to a function

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

nlohmann::json j;
std::ifstream if( "C:\\example.json" );
j << if;
if.close( );

/*...*/

void square( int *in )
{
    in *= in;
}

/*...*/

square( &j[ "int" ] );

• What is the expected behavior?
  To be able to pass the object by reference, rather than this:

int buf = j[ "int" ];
square( &buf );
j[ "int" ] = buf;

• And what is the actual behavior instead?
  Error (active) E0167 argument of type "nlohmann::basic_json<std::map, std::vector, std::string, bool, int64_t, uint64_t, double, std::allocator, nlohmann::adl_serializer> *" is incompatible with parameter of type "int *"

• Which compiler and operating system are you using? Is it a supported compiler?
  Visual Studio 2017 15.5.3, Windows 10

• Did you use a released version of the library or the version from the develop branch?
  Released

• If you experience a compilation error: can you compile and run the unit tests?
  Yes

[c-m-w]

+1

[pboettch]

One problem is that you're mixing references and pointers. Your square-function expects a reference to in int (int &) but you're passing a pointer (&j["int"]). (The same is true for your "rather than this"-example, which won't work either).

[nlohmann]

You can use the get_ref function to get a reference and the get_ptr function to get a point to a stored value.

However, the library does not store integers as int, but uses uint64_t for unsigned values and int64_t for signed values. You can check the value with is_number_unsigned() and is_number_integer(), respectively. In either case, get_ref<int&>() or get_ptr<int*>() will not work.

You could template your square function to work with any number reference type, and then pass a reference to uint64_t or int64_t. It could be even easier to pass json & and directly manipulate the value.

[nlohmann]

@J-e-r-e-m-i-a-h Do you need further assistance?

The example is wrong, by the way; the square function is improper (I made the post late at night). I just copied the parameters for a Windows function I'm using and then simplified it in order to post an example. Either way, I do require passing a variable by reference like &i for int i as a parameter to the function.

As of now, I'm trying to figure it out, I'll let you know if I can't figure out how to use get_ref :P I'm pretty sure your proposed solution is correct. Here's what I've got so far, let me know if I'm on the right track :)

if ( j[ "int" ].is_number_unsigned( ) )
    square( j[ "int" ].get_ref<uint64_t&>( ) );
else if ( j[ "int" ].is_number_integer( ) )
    square( j[ "int" ].get_ref<int64_t&>( ) );

[nlohmann]

Looks right.

All is good then! Thanks for the help! I love this resource, absolutely amazing.