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Explicit Type Conversion Operator: ()

C++ allows explicit type conversion using syntax similar to the function-call syntax.

simple-type-name ( expression-list )

A simple-type-name followed by an expression-list enclosed in parentheses constructs an object of the specified type using the specified expressions. The following example shows an explicit type conversion to type int:

int i = int( d );

The following example uses a modified version of the Point class defined in Function-Call Results.

// expre_Explicit_Type_Conversion_Operator.cpp
// compile with: /EHsc
#include <iostream>

using namespace std;
class Point
{
public:
    // Define default constructor.
    Point() { _x = _y = 0; }
    // Define another constructor.
    Point( int X, int Y ) { _x = X; _y = Y; }

    // Define "accessor" functions as
    // reference types.
    unsigned& x() { return _x; }
    unsigned& y() { return _y; }
    void Show()   { cout << "x = " << _x << ", "
                         << "y = " << _y << "\n"; }
private:
    unsigned _x;
    unsigned _y;
};

int main()
{
    Point Point1, Point2;

    // Assign Point1 the explicit conversion
    //  of ( 10, 10 ).
    Point1 = Point( 10, 10 );

    // Use x() as an l-value by assigning an explicit
    //  conversion of 20 to type unsigned.
    Point1.x() = unsigned( 20 );
    Point1.Show();

    // Assign Point2 the default Point object.
    Point2 = Point();
    Point2.Show();
}

x = 20, y = 10
x = 0, y = 0

Although the preceding example demonstrates explicit type conversion using constants, the same technique works to perform these conversions on objects. The following code fragment demonstrates this:

int i = 7;
float d;

d = float( i );

Explicit type conversions can also be specified using the "cast" syntax. The previous example, rewritten using the cast syntax, is:

d = (float)i;

Both cast and function-style conversions have the same results when converting from single values. However, in the function-style syntax, you can specify more than one argument for conversion. This difference is important for user-defined types. Consider a Point class and its conversions:

struct Point
{
    Point( short x, short y ) { _x = x; _y = y; }
    ...
    short _x, _y;
};
...
Point pt = Point( 3, 10 );

The preceding example, which uses function-style conversion, shows how to convert two values (one for x and one for y) to the user-defined type Point.

Caution noteCaution

Use the explicit type conversions with care, since they override the C++ compiler's built-in type checking.

The cast notation must be used for conversions to types that do not have a simple-type-name (pointer or reference types, for example). Conversion to types that can be expressed with a simple-type-name can be written in either form. See Type Specifiers for more information about what constitutes a simple-type-name.

Type definition within casts is illegal.

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