Using Function Pointers

When we use the name of a function as a value, the function is automatically converted to a pointer. For example, we can assign the address of lengthCompare to pf as follows:

pf = lengthCompare;  // pf now points to the function named lengthCompare
pf = &lengthCompare; // equivalent assignment: address-of operator is optional

Moreover, we can use a pointer to a function to call the function to which the pointer points. We can do so directly—there is no need to dereference the pointer:

bool b1 = pf("hello", "goodbye");    // calls lengthCompare
bool b2 = (*pf)("hello", "goodbye"); // equivalent call
bool b3 = lengthCompare("hello", "goodbye"); // equivalent call

There is no conversion between pointers to one function type and pointers to another function type. However, as usual, we can assign nullptr (§ 2.3.2, p. 53) or a zero-valued integer constant expression to a function pointer to indicate that the pointer does not point to any function:

string::size_type sumLength(const string&, const string&);
bool cstringCompare(const char*, const char*);
pf = 0;              // ok: pf points to no function
pf = sumLength;      // error: return type differs
pf = cstringCompare; // error: parameter types differ
pf = lengthCompare;  // ok: function and pointer types match exactly