C/C++, function pointer

0. A function pointer is a variable that stores the address of a function that will be called later through that function pointer. 

    Function pointers are among the most powerful tools in C.  It can be used to implement function callback in C. C++ takes a slightly different route for callbacks. 

 // (1)define a function

bool lengthCompare(const string&, const string&){

　　return true;

}

// (2)define a function pointer and initialize it 

bool (*pf) (const string&, const string&); 

pf = lengthCompare;

pf = &lengthCompare;

// (3)call a function through a function pointer

cmpFcn pf = lengthCompare;  // initialize the pf

pf("hi","bye"); // implicitly dereferenced 

(*pf)("hi","type"); // explicitly dereferenced 

// (4)define a type named cmpFcn, this type is a pointer to function.  

typedef bool (*cmpFcn) (const string&, const string&);

typedef int func(int*, int); // func is a function type, not a pointer to a function.

// (5)function pointer as a parameter

void useBigger(const string&, const string&, bool (*)(const string&, const string&));

void useBigger(const string&, const string&, bool (const string&, const string&));

// (6)function pointer as a return type

int (*ff(int))(int*, int); // is the same as following codes

typedef int (*pf)(int&, int);

pf ff(int); // ff return a pointer to function

// (7)example

 int main(){

　　cmpFcn pf1;
　　cmpFcn pf2 = 0; 
　　cmpFcn pf3 = lengthCompare;
　　cmpFcn pf4 = &lengthCompare;

　　pf = lengthCompare;
　　cout << pf << endl;　　 // 1
　　cout << pf1 << endl;　　// 0
　　cout << pf2 << endl;　　// 0
　　cout << pf3 << endl;　　// 1
　　cout << pf4 << endl;　　// 1

　　return 0;
}

2. There is no conversation between one pointer to function type and another. 

A function pointer must point to the function whose type is exactly the same as this pointer points to.

3. Benefits of function pointers

    Write flexible functions and libraries that allow the programmer to choose behavior by passing function pointers as arguments.

    This flexibility can also be achieved by using classes with virtual functions.

( Most of the code are coming from C++ primer fourth edition  )