父类中的构造方法不会将值分配给私有变量

Question

This is my code and you can also run it from http://cpp.sh/5lsds

#include "iostream"
using namespace std;
class X{
private:
    int c;
public:
    X(){}
    X(int b){
    c = 11;
    }
    int getC();
};
class Z:public X{
public:
    Z(int n){
        X(23);
    }
};
int main()
{
    Z z(1);
    cout<<z.getC()<<endl;
    return 0; 
}
int X::getC(){
    return c;
}

I need to have X(){} line since the child constructor needs to call the parent default constructor.

If you run the program from http://cpp.sh/5lsds you can see that the output is 0 while I expect it to be 11 . Since the Z constructor calls X constructor with an int parameter and it sets the c value to 11 but the output is 0 .

Answer 1

You should use member initializer list ,

In the definition of a constructor of a class, member initializer list specifies the initializers for direct and virtual base subobjects and non-static data members.

eg

Z(int n) : X(23) {}

I need to have X(){} line since the child constructor needs to call the parent default constructor.

With member intializer list it's not required again (in this code sample).

For X(23); in the body of the constructor, you're just creating a temporary X , which has nothing to do with the base subobject X of Z ; Then the default constructor of X (ie X::X() ) will be used for it. ie it's equivalent with:

Z(int n) : X() {  // initialize the base suboject X via X::X()
    X(23);        // create an unnamed temporary via X::X(int)
}

Answer 2

You don't invoke the constructor of the base class

Z(int n){
    X(23);
}

This creates an unnamed temporary X object, and passes 23 to its constructor. It doesn't construct the X sub-object of Z.

In C++, we construct bases and members using the member initializer list syntax:

X(int b) :
  c(11)
{}

Z(int n) :
  X(23)
{}

The member initializer list syntax is pretty much equivalent to the assignment you do when a simple integer is the constructed member. But beware that more complex sub-objects will be default constructed first, and then have their assignment operator invoked. That could make a substantial (worsening) difference in performance to just specifying them in the member initializer list and constructing once.