Object conversions return the specific type that was converted.

[davidchappelle]

This allows us to easily make function calls that have parameters that are dependent
upon converting from msgpack objects to concrete types. For example:

void process_args(std::tuple<int,std::string>& args)
{
    ...
}

process_args(obj.convert(std::make_tuple(100,"hello")))

You can get similar behavior by using obj.as<...>() but its performance is highly
dependent upon the specific compiler and whether or not r-value references are
supported.

[redboltz]

Thank you for sending the PR.
It seems that the following code wouldn't compiled successfully.

obj.convert(std::make_tuple(100,"hello"))

The member function 'convert' has a reference parameter. It is a what we call out parameter.

inline T& object::convert(T& v) const

std::make_tuple(100,"hello") creates a temporary rvalue object. That kind of object cannot be passed as a lvalue reference parameter.

Could you check your code example and the contents of PR?

[davidchappelle]

Apologies for the bad example. The existing code has the same issue regardless of my patch. Here is an updated example:

#include <iostream>
#include <msgpack.hpp>
#include <string>
#include <tuple>

void process_args(std::tuple<int,std::string>& args)
{
    std::cout << "processing args" << std::endl;
}

int main(int argc, char** argv)
{
    msgpack::object obj;
    std::tuple<int, std::string> t;
    process_args(obj.convert(t));

    return 0;
}

[davidchappelle]

Sounds reasonable?

[redboltz]

Yes. We can write as follows:

    std::tuple<int, std::string> t;
    process_args(obj.convert(t));

instead of

    std::tuple<int, std::string> t;
    obj.convert(t);
    process_args(t);

I think it is a same mannar as memcpy.
http://www.cplusplus.com/reference/cstring/memcpy/

I will merge your PR. Before I do that though, could you add some tests?

I think that the following file is an appropriate location:
https://github.com/msgpack/msgpack-c/blob/master/test/convert.cpp

[redboltz]

I added tests and merged.

[redboltz]

This fix includes very subtle breaking change.
If the client code has the following code:

void foo() {
    msgpack::object obj;
    int i;
    return obj.convert(i); // void function can return 

a compile would report the error like "cannot convert from int& to void."
However, it is very rare that returning void return code explicitly.