如何在另一个类中调用一个类的构造函数？

Question

I have a class first in which I want to have another element that its type is another class. Something like this:

class first{

private:

    second secondAvl;
public:

    first():second(-1){}  // i get erroe here
} 

class second: public Tree{

private:

public:
 second(int key) :Tree(NULL,key1){} // here it worked to call contructor for tree
}

My problem is that when I try to call the constructor for second in class first constructor I get this error:

no matching function for call to 'second::second()'

Any help what I am doing wrong? Because I did the same thing when I called the constructor for tree in the second class and that worked fine.

Answer 1

First, in the order you define the classes, class second is not known at the time it is used in first . You should actually get other error messages. Second, in the initializer list, you need to address the variable to initialize by its name (ie : secondAvl(-1) ), not by its type : second(-1) .

See the following working example:

class second {

private:

public:
    second(int key) {} // here it worked to call contructor for tree
};


class first{

private:

    second secondAvl;
public:

    first():secondAvl(-1){}  // i get erroe here
};

Answer 2

do instead:

...

private:

    second secondAvl;
public:

    first() : secondAvl(-1)
    { }  
}

or uniform initialization using {}

...

private:

    second secondAvl;
public:

    first() : secondAvl{-1}
    { }  
}