Function Call Operator: ()

 

A postfix-expression followed by the function-call operator, ( ), specifies a function call.


postfix-expression 
( [argument-expression-list ] )

The arguments to the function-call operator are zero or more expressions separated by commas — the actual arguments to the function.

The postfix-expression must evaluate to a function address (for example, a function identifier or the value of a function pointer), and argument-expression-list is a list of expressions (separated by commas) whose values (the arguments) are passed to the function. The argument-expression-list argument can be empty.

The postfix-expression must be of one of these types:

  • Function returning type T. An example declaration is

    T func( int i )
    
  • Pointer to a function returning type T. An example declaration is

    T (*func)( int i )
    
  • Reference to a function returning type T. An example declaration is

    T (&func)(int i)
    
  • Pointer-to-member function dereference returning type T. Example function calls are

    (pObject->*pmf)();
    (Object.*pmf)();
    

Example

The following example calls the standard library function strcat_s with three arguments:

// expre_Function_Call_Operator.cpp
// compile with: /EHsc

#include <iostream>
#include <string>

// STL name space
using namespace std;

int main()
{
    enum
    {
        sizeOfBuffer = 20
    };

    char s1[ sizeOfBuffer ] = "Welcome to ";
    char s2[ ] = "C++";

    strcat_s( s1, sizeOfBuffer, s2 );

    cout << s1 << endl;
}
Welcome to C++

A function call evaluates to an r-value unless the function is declared as a reference type. Functions with reference return type evaluate to l-values, and can be used on the left side of an assignment statement as follows:

// expre_Function_Call_Results.cpp
// compile with: /EHsc
#include <iostream>
class Point
{
public:
    // Define "accessor" functions as
    // reference types.
    unsigned& x() { return _x; }
    unsigned& y() { return _y; }
private:
    unsigned _x;
    unsigned _y;
};

using namespace std;
int main()
{
    Point ThePoint;

    ThePoint.x() = 7;           // Use x() as an l-value.
    unsigned y = ThePoint.y();  // Use y() as an r-value.

    // Use x() and y() as r-values.
    cout << "x = " << ThePoint.x() << "\n"
         << "y = " << ThePoint.y() << "\n";
}

The preceding code defines a class called Point, which contains private data objects that represent x and y coordinates. These data objects must be modified and their values retrieved. This program is only one of several designs for such a class; use of the GetX and SetX or GetY and SetY functions is another possible design.

Functions that return class types, pointers to class types, or references to class types can be used as the left operand to member-selection operators. Therefore, the following code is legal:

// expre_Function_Results2.cpp
class A {
public:
   A() {}
   A(int i) {}
   int SetA( int i ) {
      return (I = i);
   }

   int GetA() {
      return I;
   }

private:
   int I;
};

A func1() {
   A a = 0;
   return a;
}

A* func2() {
   A *a = new A();
   return a;
}

A& func3() {
   A *a = new A();
   A &b = *a;
   return b;
}

int main() {
   int iResult = func1().GetA();
   func2()->SetA( 3 );
   func3().SetA( 7 );
}

Functions can be called recursively. For more information about function declarations, see Function Specifiers and Member Functions. Related material is in Program and Linkage.

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