function pointer parameter with type alias

Question

im trying some examples in a book(c++ primer by lippman) and im trying to learn about function pointers

this code:

#include <iostream>

void useBigger (const std::string &s1, const std::string &s2,
            bool (*func)(const std::string &, const std::string &))
            {
                bool valid = func (s1, s2);
                std::cout << __func__ << " is called "
                          << valid <<std::endl;
            }

bool lengthCompare (const std::string &s1, const std::string &s2)
{
if (s1.size() > s2.size())
    return true;
else
    return false;
}

int main()
{
useBigger ("hello", "sample", lengthCompare);


return 0;
}

this code just runs fine but when i try using type alias such as typedef

#include <iostream>

typedef bool func (const std::string &, const std::string &); /// or typedef bool (*func)(const std::string &, const std::string);

void useBigger (const std::string &s1, const std::string &s2,
            func)
            {
                bool valid = func (s1, s2);
                std::cout << __func__ << " is called "
                          << valid <<std::endl;
            }

bool lengthCompare (const std::string &s1, const std::string &s2)
{
if (s1.size() > s2.size())
    return true;
else
    return false;
}

int main()
{
useBigger ("hello", "hiiiii", lengthCompare);


return 0;
}

it produces an error something like this:

  error: expression list treated as compound expression in functional cast [-fpermissive]

Answer 1

The symbol func is a type alias , but you use it as a function. You need to actually declare an argument variable and use it instead of the type, like eg

void useBigger (const std::string &s1, const std::string &s2,
                func f)
        {
            bool valid = f (s1, s2);
            std::cout << __func__ << " is called "
                      << valid <<std::endl;
        }

Answer 2

Your type defenition need to correct as follows:

From

typedef bool func (const std::string &, const std::string);

TO

typedef bool func (const std::string &, const std::string&);

And in function useBigger you have to pass the function type with variable name and need to correct function definition as follows:

void useBigger (const std::string &s1, const std::string &s2,
            func f)
            {
                bool valid = f (s1, s2);
                std::cout << __func__ << " is called "
                          << valid <<std::endl;
            }