How to Call the Base Class Constructor in C++

c++constructor

Lately, I have done much programming in Java. There, you call the class you inherited from with super(). (You all probably know that.)

Now I have a class in C++, which has a default constructor which takes some arguments. Example:

class BaseClass {
public:
    BaseClass(char *name); ....

If I inherit the class, it gives me the warning that there is no appropriate default constructor available. So, is there something like super() in C++, or do I have to define a function where I initialize all variables?

Best Answer

You do this in the initializer-list of the constructor of the subclass.

class Foo : public BaseClass {
public:
    Foo() : BaseClass("asdf") {}
};

Base-class constructors that take arguments have to be called there before any members are initialized.

Related Solutions

C++ Inheritance – Rules for Calling the Base Class Constructor

Base class constructors are automatically called for you if they have no argument. If you want to call a superclass constructor with an argument, you must use the subclass's constructor initialization list. Unlike Java, C++ supports multiple inheritance (for better or worse), so the base class must be referred to by name, rather than "super()".

class SuperClass
{
    public:

        SuperClass(int foo)
        {
            // do something with foo
        }
};

class SubClass : public SuperClass
{
    public:

        SubClass(int foo, int bar)
        : SuperClass(foo)    // Call the superclass constructor in the subclass' initialization list.
        {
            // do something with bar
        }
};

More info on the constructor's initialization list here and here.

Why Base Class Constructor is Called in Derived Type – C++

The short answer for this is, "because that's what the C++ standard specifies".

Note that you can always specify a constructor that's different from the default, like so:

class Shape  {

  Shape()  {...} //default constructor
  Shape(int h, int w) {....} //some custom constructor


};

class Rectangle : public Shape {
  Rectangle(int h, int w) : Shape(h, w) {...} //you can specify which base class constructor to call

}

The default constructor of the base class is called only if you don't specify which one to call.

Best Answer

Related Solutions

C++ Inheritance – Rules for Calling the Base Class Constructor

Why Base Class Constructor is Called in Derived Type – C++

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