Question

I have defined class template named CallBackAtInit which only purpose is to call a function at its initialization (constructor). The function is specified in template parameters. The problem is that templates does not accept std::function as parameters; but they accept function pointers. Why?

Here is my code:

#include <iostream>
#include <functional>

/* Does not work:*/     template <typename return_type, typename arg_type, std::function<return_type(arg_type)> call_back>
/* Work fine: *///      template <typename return_type, typename arg_type, return_type(*call_back)(arg_type)>

class CallBackAtInit {
public:
    CallBackAtInit(arg_type arg)
    {
        call_back(arg);
    };
};

void printInt(int i);

class HoldInt : private CallBackAtInit<void, int, printInt> {
public:
    HoldInt(int integer)
    : CallBackAtInit(integer)
    {}
    // ...
};

int main(int argc, char** argv)
{
    HoldInt hi(10);
    return 0;
}

void printInt(int i)
{
    std::cout << i << std::endl;
}

Answer 1

The parameter for a template definition can be of four kinds:

• parameter which can accept only type (or template type).
• parameter which can accept only integral value.
• parameter which can accept only pointer-to-member value.
• std::nullptr_t (since C++11)

When you mention std::function in a template definition, then it falls neither of the above categories. The template cannot accept types, nor can it accept integral value, or pointer-to-member value.

When the parameter is function pointer type, then it can accept function-pointer (the address of a function matching the type) which is just an integral value. Note that address is always an integral value. So it falls into the second category, which is why it works.