Question

I’m trying to learn c++ from the basics, and I was playing around with function pointers. Considering this code:

#include <iostream>
#include <string>
#include <vector>

bool print(std::string);
bool print(std::string a) 
{
    std::cout << a << std::endl;
    return true;
}

bool call_user_function(bool(std::string), std::vector<std::string>);
bool call_user_function(bool(*p)(std::string), std::vector<std::string> args) {
    if (args.size() == 0)
        return (*p)();                    (*)
    else if (args.size() == 1)
        return (*p)(args[0]);
    else if (args.size() == 2)
        return (*p)(args[0], args[1]);    (**)
}

int main(int argc, char** argv) 
{
    std::vector<std::string> a;
    a[0] = "test";
    call_user_function(print, a);
    // ok
    return 0;
}

It gives me:

main.cpp:28 (*): error: too few arguments to function

main.cpp:32 (**): error: too many arguments to function

What am I doing wrong?

Answer 1

p is of type bool(*)(std::string). This means it is a pointer to a function that has a single parameter of type std::string and returns a bool.

p can point to print, because the type of print matches: it is a function that has a single parameter of type std::string and returns a bool.

Your first erroneous expression, (*p)(), attempts to call p with no arguments. Your second erroneous expression, (*p)(args[0], args[1]) attempts to call p with two arguments.

The number of arguments must match the number of parameters, so both of these are ill-formed, just like an attempt to call print directly with no arguments or with two arguments would result in a compilation error.