usage on c++ function pointer - c++

Question

I'm a newbie to C++, learning pointer of function recently, a little confused by usage of pointer of function;
I practiced the following code:
#include <iostream>
#include <sstream>
using namespace std;
int subtraction(int a,int b){
return a-b;
}
int main(int argc, const char * argv[])
{
int (*minus)(int,int)=subtraction;
cout<<minus(5,4);
return 0;
}
it works well;
so,I try a little variation:
#include <iostream>
#include <sstream>
using namespace std;
int subtraction(int a,int b){
return a-b;
}
int main(int argc, const char * argv[])
{
int *minus(int,int)=subtraction;//only here different!
cout<<minus(5,4);
return 0;
}
I practiced it in Xcode on Mac,it give me Error:
Illegal initializer (only variables can be initialized)
but I think compiler can recognized the two is same,why must have a pair of parenthesizes?

Answer 1

In your original code
int (*minus)(int,int)=subtraction;
declares minus as a function pointer that takes parameter int, int and returns int.
In your second code
int *minus(int,int)=subtraction;
declares minus as a function that takes parameter int, int and returns a pointer int *.
You can use a function name(which is automatically converted to a function pointer) to initialize a function pointer, but you can't initialize a function.

Answer 2

This is a matter of operator precedence. The function call operator () has a higher precedence than the dereference operator *. So you must use parentheses to specify the correct order of evaluation.
int *minus(int, int)
means: First call a function named minus, then dereference the return value (int* in this case).
int (*minus)(int, int)
means: First dereference "minus", which returns a function, and then call that function.

Answer 3

You have tagged your code C++ and using iostream so I can safely assume you are looking for a C++ solution.
In such scenario, its best to use class template std::function instead of the function pointer syntax that is prone to error.
#include <iostream>
#include <sstream>
#include <functional>
int subtraction(int a,int b){
return a-b;
}
int main(int argc, const char * argv[])
{
std::function<int(int,int)> minus = subtraction;
//int (*minus)(int,int)=subtraction;
std::cout<<minus(5,4);
return 0;
}
Alternatively, if you would still want to continue with pointer to function, typedefs are recommended
#include <iostream>
int subtraction(int a,int b){
return a-b;
}
typedef int (*MINUS)(int,int);
int main(int argc, const char * argv[])
{
MINUS minus = subtraction;
//int (*minus)(int,int)=subtraction;
std::cout<<minus(5,4);
return 0;
}
And finally, another widely used option is to use functors.
#include <iostream>
struct MINUS
{
int operator()(int a,int b){
return a-b;
}
};
int main(int argc, const char * argv[])
{
//int (*minus)(int,int)=subtraction;
MINUS minus;
std::cout<<minus(5,4);
return 0;
}