Create a default constructor in C++

Question

This might be a stupid question but I can't find a lot of information on the web about creating your own default constructors in C++. It seems to just be a constructor with no parameters. However, I tried to create my default constructor like this:

Tree::Tree()  {root = NULL;}

I also tried just:

Tree::Tree() {}

When I try either of these I am getting the error:

No instance of overloaded function "Tree::Tree" matches the specified type.

I can't seem to figure out what this means.

I am creating this constructor in my .cpp file. Should I be doing something in my header (.h) file as well?

Answer 1

Member functions (and that includes constructors and destructors) have to be declared in the class definition:

class Tree {
public:
    Tree(); // default constructor
private:
    Node *root;

};

Then you can define it in your .cpp file:

Tree::Tree() : root(nullptr) {
}

I threw in the nullptr for C++11. If you don't have C++11, use root(0).

Answer 2

C++11 allows you to define your own default constructor like this:

class A {  
    public:
        A(int);        // Own constructor
        A() = default; // C++11 default constructor creation
};

A::A(int){}

int main(){
    A a1(1); // Okay since you implemented a specific constructor
    A a2();  // Also okay as a default constructor has been created
}

Answer 3

It isn't sufficient to create a definition for any member function. You also need to declare the member function. This also applies to constructors:

class Tree {
public:
    Tree(); // declaration
    ...
};

Tree::Tree() // definition
    : root(0) {
}

As a side note, you should use the member initializer list and you should not use NULL. In C++ 2011 you want to use nullptr for the latter, in C++ 2003 use 0.

Answer 4

Yes, you need to declare it in your header. For example place the following inside the declaration of the tree class.

class Tree {
    // other stuff...
    Tree();
    // other stuff...
};