C ++构造函数调用另一个构造函数

Question

When attempting to build a c++ program containing the following code:

menutype::menutype(int cat_num){
    extras list = extras(cat_num);
}

extras::extras(int num_cats){
    head = new category_node;
    head->next = NULL;
    head->category = 1;
    category_node * temp;
    for(int i = 1; i < (num_cats); ++i){
        temp = new category_node;
        temp->next = head->next;
        head->next = temp;
        temp->category = (num_cats-(i-1));
    }
}

I receive the error :

cs163hw1.cpp: In constructor 'menutype::menutype(int)':
cs163hw1.cpp:59:31: error: no matching function for call to 'extras::extras()'
cs163hw1.cpp:59:31: note: candidates are:
cs163hw1.cpp:5:1: note: extras::extras(int)

And I do not understand why, please help!

Answer 1

Since that line shouldn't attempt to call a default constructor (only copy constructor and conversion constructor from int ), I'll just guess that you have a data member of type extras in your class menutype , so you have to initialize it in the initializer list because it doesn't have a default constructor:

menutype::menutype(int cat_num) : list(cat_num) { //or whatever the member is called

}

Answer 2

It seems like your menutype holds a member of type extras . If that is the case, and if extras does not have a default constructor (as it seems to be the case) you need to initialize it in the initialization list:

menutype::menutype(int cat_num) : myextrasmember(cat_num) {}

Answer 3

One would typically call a constructor within a constructor of another class (as in your example) the following way:

menutype::menutype(int cat_num) : list(cat_num) { }

This is more efficient as the constructor for list (of type extra) is called in the initialiser list.

Answer 4

As others said, you are calling the constructor incorrectly.

Three other people have already pointed out the proper method. However, no one has pointed out how to properly call a constructor outside of a constructor context.

Instead of:

extras list = extras(cat_num);

Do:

extras list(cat_num);