Lodec Lodec - 3 years ago 152
C++ Question

Understand some specific cases of the constructor

I have the following code :

#include <iostream>
using namespace std;
class X {
public:
X () {
x = 0;
cout << "1";
}
X (int x) {
this->x = x;
cout << "2";
}
X (double x) {
this->x = x;
cout << "3";
}
X (const X& x) {
this->x = x.x;
cout << "4";
}
protected:
int x;
};
class Y : public X {
public:
Y () : X(10) {
cout << "5";
}
Y (int p) : X(p) {
cout << "6";
}
Y (const X& x) : X(x) {
cout << "7";
}
Y (const X& x1, X x2) : X(x1), x(x2) {
cout << "8";
}
protected:
X x;
};

int main() {
Y y1;
cout << endl;
Y y2(10);
cout << endl;
Y y3(y1);
cout << endl;
Y y4(y2, y3);
cout << endl;
}


The output of this code is :

215
216
44
4448



  • In the first two cases, I don't understand why there is the '1'. I agree that the object is built with the second X constructor and the first of Y but why the first constructor of X is called after the second one ?

  • In the third case I would like to know why there is no '7' because in my opinion the program declares a Y instance so a Y constructor should be called ?

  • In the last case, there is a '8' so that sound good to me but why there is three X constructor call whereas y2 and y3 are already declared ?



PS : I apologize for my English mistakes and I thank the people who correct me

Answer Source

Y derives from X, and also contains a data member x of type X. When instantiating an object instance of Y, an X constructor must be called for both the inherited X portion of the Y class, and the x data member.

In the first 2 cases, the Y constructors you are calling are not explicitly initializing the x data member, so the compiler uses the default X() constructor to initialize it implicitly. That is where the 1 comes from.

In the 3rd case, 7 is not output because you are not constructing the y3 object with an X object as input. You are constructing it with another Y object. But you haven't explicitly defined a Y(const Y&) copy constructor, so the compiler defines an implicit copy constructor for you, which invokes the X copy constructor twice, once for the inherited X portion of the Y class, and once for the x data member.

In the last case, you see 4 being output 3 times because you are invoking the X copy constructor 3 times, two explicitly and one implicitly:

  • when passing x1 to X()
  • when passing y3 to x2 (which slices y3, because x2 is pass-by-value)
  • when passing x2 to x()
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