Explicit Type Conversion Operator: ()
C++ allows explicit type conversion using syntax similar to the function-call syntax.
simple-type-name ( expression-list )
Remarks
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.
Example
// 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();
}
Output
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.
Aviso
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.