std::bind null function pointer

Question

#include <iostream>
#include <functional>

typedef int(*SumFptr)(int,int);

int main()
{

    SumFptr ptr = nullptr;
    std::function<int(int)> func = std::bind(ptr, std::placeholders::_1, 42);
    std::cout << (func ? "true" : "false") << std::endl;

    return 0;
}

The output is true.

My expectation was false, like std::function{nullptr}. Is it a bug or correct behaviour and where can I read about it?

Answer 1

How is the std::function supposed to know that the object it was constructed with contains a null pointer?

As far as it's concerned it has a callable object, it can't know that it is a callable object containing a null pointer (which would crash if called).

When you construct std::function{nullptr} it knows it doesn't have a callable object, and behaves exactly as if you'd called the default constructor. std::function{ anything_else } causes it to store anything_else and unless it is a null pointer (not an object that wraps a null pointer!) will consider itself to have a target object.

http://en.cppreference.com/w/cpp/utility/functional/function/operator_bool

std::function::operator bool
Return value: true if *this stores a callable function target, false otherwise.

Put another way, your program is almost exactly equivalent to this:

#include <iostream>
#include <functional>

using SumFPtr = int(*)(int,int);

struct BoundFunction {    
    SumFptr fptr;
    int y;

    int operator()(int x) const { return fptr(x, y); }
};

BoundFunction bind(SumFPtr fptr, int y)
{
    return BoundFunction{fptr, y};
}

int main()
{
    std::function<int(int)> func = bind(nullptr, 42);
    std::cout << (func ? "true" : "false") << std::endl;
}

Now it should be obvious that of course it prints "true", because the std::function contains a callable object.