Named Constructor

More C++ Idioms Named Constructor

isocpp What is the “Named Constructor Idiom”? ¶ Δ

A technique that provides more intuitive and/or safer construction operations for users of your class.

#include <cmath>               // To get std::sin() and std::cos()

/*
 // 反面例子
 class Point {
 public:
 Point(float x, float y);     // Rectangular coordinates
 Point(float r, float a);     // Polar coordinates (radius and angle)
 // ERROR: Overload is Ambiguous: Point::Point(float,float)
 };
 int main()
 {
 Point p = Point(5.7, 1.2);   // Ambiguous: Which coordinate system?
 // ...
 }

 */
class Point
{
public:
    static Point rectangular(float x, float y);      // Rectangular coord's 直角坐标系
    static Point polar(float radius, float angle);   // Polar coordinates 极坐标系
    // These static methods are the so-called "named constructors"
    // ...
private:
    Point(float x, float y);     // Rectangular coordinates
    float x_, y_;
};
inline Point::Point(float x, float y)
        : x_(x), y_(y)
{
}
inline Point Point::rectangular(float x, float y)
{
    return Point(x, y);
}
inline Point Point::polar(float radius, float angle)
{
    return Point(radius * std::cos(angle), radius * std::sin(angle));
}
int main()
{
    Point p1 = Point::rectangular(5.7, 1.2);   // Obviously rectangular
    Point p2 = Point::polar(5.7, 1.2);         // Obviously polar
    // ...
}