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Function Call (C++)

The function-call operator, invoked using parentheses, is a binary operator.

primary-expression ( expression-list )

In this context, primary-expression is the first operand, and expression-list, a possibly empty list of arguments, is the second operand. The function-call operator is used for operations that require a number of parameters. This works because expression-list is a list instead of a single operand. The function-call operator must be a nonstatic member function.

The function-call operator, when overloaded, does not modify how functions are called; rather, it modifies how the operator is to be interpreted when applied to objects of a given class type. For example, the following code would usually be meaningless:

Point pt;
pt( 3, 2 );

Given an appropriate overloaded function-call operator, however, this syntax can be used to offset the x coordinate 3 units and the y coordinate 2 units. The following code shows such a definition:

// function_call.cpp
class Point
{
public:
    Point() { _x = _y = 0; }
    Point &operator()( int dx, int dy )
        { _x += dx; _y += dy; return *this; }
private:
    int _x, _y;
};

int main()
{
   Point pt;
   pt( 3, 2 );
}

Note that the function-call operator is applied to the name of an object, not the name of a function.

You can also overload the function call operator using a pointer to a function (rather than the function itself).

typedef void(*ptf)();
void func()
{
}
struct S
{
   operator ptf()
   {
      return func;
   }
};

int main()
{
   S s;
   s();//operates as s.operator ptf()()
}
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