Question

I am trying to write a template function that accepts a std::function which depends on the template arguments. Unfortunately the compiler is not capable of correctly deucing the arguments to the std::function. Here some simple example code:

#include <iostream>
#include <functional>

using namespace std;

void DoSomething( unsigned ident, unsigned param )
{
    cout << "DoSomething called, ident = " << ident << ", param = "  << param << "\n";
}

template < typename Ident, typename Param >
void CallFunc( Ident ident, Param param, std::function< void ( Ident, Param ) > op )
{
    op( ident, param );
}

int main()
{
    unsigned id(1);
    unsigned param(1);

    // The following fails to compile
    // CallFunc( id, param, DoSomething );

    // this is ok 
    std::function< void ( unsigned, unsigned ) > func( DoSomething );
    CallFunc( id, param, func ); 

    return 0;
}

If I call the template with the following:

CallFunc( id, param, DoSomething );

I get the following errors:

function-tpl.cpp:25: error: no matching function for call to CallFunc(unsigned int&, unsigned int&, void (&)(unsigned int, unsigned int))

If I explicitly create a std::function of the correct type (or cast it) the problem goes away:

std::function< void ( unsigned, unsigned ) > func( DoSomething );
CallFunc( id, param, func );

How would I code this so that the explicit temporary is not needed?

Answer 1

If you are using templates, you can avoid std::function entirely, unless for some reason you want to specifically restrict the function to take std::function:

template < typename Ident, typename Param, typename Func >
void CallFunc( Ident ident, Param param, Func op )
{
    op( ident, param );
}