How to make template rvalue reference parameter ONLY bind to rvalue reference?

Question

I\'m writing a network library and use move semantics heavily to handle ownership for file descriptors. One of my class wishes to receive file descriptor wrappers of other k

Answer 1

A simple way is to provide a deleted member which accepts an lvalue reference:

template void receive_ownership(T&) = delete;

This will always be a better match for an lvalue argument.

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If you have a function that takes several arguments, all of which need to be rvalues, we will need several deleted functions. In this situation, we may prefer to use SFINAE to hide the function from any lvalue arguments.

One way to do this could be with C++17 and the Concepts TS:

#include 

template
void receive_ownership(T&& t)
    requires !std::is_lvalue_reference::value
{
     // taking file descriptor of t, and clear t
}

or

#include 

void receive_ownership(auto&& t)
    requires std::is_rvalue_reference::value
{
     // taking file descriptor of t, and clear t
}

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Going slightly further, you're able to define a new concept of your own, which may be useful if you want to reuse it, or just for extra clarity:

#include 

template
concept bool rvalue = std::is_rvalue_reference::value;


void receive_ownership(rvalue&& t)
{
     // taking file descriptor of t, and clear t
}

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Note: with GCC 6.1, you'll need to pass -fconcepts to the compiler, as it's an extension to C++17 rather than a core part of it.

Just for completeness, here's my simple test:

#include 
int main()
{
    int a = 0;
    receive_ownership(a);       // error
    receive_ownership(std::move(a)); // okay

    const int b = 0;
    receive_ownership(b);       // error
    receive_ownership(std::move(b)); // allowed - but unwise
}