A: A method which provides more intuitive and/or safer construction operations for users of your class.
The difficulty is that constructors have the same name always as the class. Thus the only way to differentiate among the various constructors of a class is via the parameter list. But if there are many constructors, the differences among them become somewhat and error prone and subtle.
Along the Named Constructor Idiom, you say publicly all the class''s constructors in protected or private sections, and you provide public static methods which return an object. These static techniques are "Named Constructors." usually, there is one such static method for each distinct way to construct an object.
For instance, suppose we are creating a Point class which represents a position on the X-Y plane. Turns out there are two common ways to mention a 2-space coordinate: polar coordinates (Radius+Angle), rectangular coordinates (X+Y). Unluckily the parameters for these two coordinate systems are the alike: two floats. It would create an ambiguity error in the overloaded constructors:
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?
...
}
One way to solve out this ambiguity is to employ the Named Constructor Idiom:
#include // To get sin() & cos()
class Point {
public:
static Point rectangular(float x, float y); // Rectangular coord''s static Point polar(float radius, float angle); // Polar coordinates
// These static methods are 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*cos(angle), radius*sin(angle)); }
The users of Point now have a clear & unambiguous syntax for developing Points in either coordinate system:
int main()
{
Point p1 = Point::rectangular(5.7, 1.2); // clearly rectangular
Point p2 = Point::polar(5.7, 1.2); // Obviously polar
...
}
Ensure your constructors are in protected section if you expect Point to contain derived classes.
The Named Constructor Idiom can also be utilized to make sure your objects are always created using new.
Note down that the Named Constructor Idiom, at least as implemented above, is only as fast as calling directly constructor modern compilers will not make any additional copies of your object.