C++ Error: No Matching Constructor for Initialization Of

Question

In memberwise assignment in C++, you can set the values of one object to another object of the same class. I initialize a rectangle object with some values and create another rectangle object but assign the value of the first into the second.

It's giving me an error. Why?

Rectangle.h

#ifndef RECTANGLE_H
#define RECTANGLE_H

class Rectangle {
    private:
        double length;
        double width;
    public:
        Rectangle(double, double);
        double getLength() const;
        double getWidth() const;
};

Rectangle::Rectangle(double l, double w) {
    length = l;
    width = w;
}

double Rectangle::getWidth() const { return width; }
double Rectangle::getLength() const { return length; }

#endif

Rectangle.cpp

#include <iostream>
#include "rectangle.h"
using namespace std;

int main()
{
    Rectangle box1(10.0, 10.0);
    Rectangle box2;

    cout << "box1's width and length: " << box1.getWidth() << ", " << box1.getLength() << endl;
    cout << "box2's width and length: " << box2.getWidth() << ", " << box2.getLength() << endl;

    box2 = box1;

    cout << "box1's width and length: " << box1.getWidth() << ", " << box1.getLength() << endl;
    cout << "box2's width and length: " << box2.getWidth() << ", " << box2.getLength() << endl;

    return 0;
}

Error when I compile:

skipper~/Desktop/Programming/Memberwise: g++ rectangle.cpp 
rectangle.cpp:7:12: error: no matching constructor for initialization of
      'Rectangle'
        Rectangle box1(10.0, 10.0);
                  ^    ~~~~~~~~~~
./rectangle.h:4:7: note: candidate constructor (the implicit copy constructor)
      not viable: requires 1 argument, but 2 were provided
class Rectangle {
  ^
./rectangle.h:4:7: note: candidate constructor
      (the implicit default constructor) not viable: requires 0 arguments, but 2
      were provided
1 error generated.

Accepted Answer

In the line

Rectangle box2; // no default constructor, error

you are trying to invoke the default constructor of Rectangle. The compiler does not generate such a default constructor anymore, because your Rectangle has a user defined constructor that takes 2 parameters. Therefore, you need to specify the parameters, like

Rectangle box2(0,10);

The error I get when compiling your code is:

Rectangle.cpp:8:15: error: no matching function for call to 'Rectangle::Rectangle()' Rectangle box2;

A solution is to create a default constructor for Rectangle, since it is not automatically generated anymore due to your user defined one:

Rectangle(); // in Rectangle.h

Rectangle::Rectangle(){} // in Rectangle.cpp (or Rectangle::Rectangle() = default; in C++11)

Another solution (and the preferable one, since it doesn't leave the data un-initialized) is to assign default arguments to your existing constructor.

Rectangle::Rectangle(double l = 0, double w = 0); // only in Rectangle.h

In this way, you make your class Default-Constructible.

Answer 2

A compiler generated default constructor is only generated if you have no defined constructors. You define a constructor, so if you want a default constructor you have to provide it yourself. Probably the easiest (arguably) is to provide it by using default arguments in your two argument constructor:

Rectangle(double l=0, double w=0)

Also you should use the inline keyword as shown below or you may find you get linker errors:

inline Rectangle::Rectangle(double l, double w) {
    length = l;
    width = w;
}

inline double Rectangle::getWidth() const { return width; }
inline double Rectangle::getLength() const { return length; }

C++ Error: No Matching Constructor for Initialization Of

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