In C++, constructor inheritance is a powerful feature that allows a derived class to inherit the properties and methods of a base class. However, there is a common issue that arises when using constructor inheritance: the default constructor call. In this article, we will explore this issue and provide some solutions to help you understand and overcome it.
When a derived class inherits from a base class, it automatically inherits all of the base class's constructors. This means that if the base class has a default constructor (a constructor that can be called with no arguments), the derived class will also have a default constructor. However, if the base class does not have a default constructor, the derived class must explicitly call one of the base class's constructors in its own constructor's initializer list.
Here is an example to illustrate this point:
class Base {
public:
Base(int x) : m\_x(x) {}
private:
int m\_x;
};
class Derived : public Base {
public:
Derived(int x, int y) : Base(x), m\_y(y) {}
private:
int m\_y;
};
In this example, the base class Base has a constructor that takes an integer argument, but it does not have a default constructor. Therefore, the derived class Derived must explicitly call the base class's constructor in its own constructor's initializer list. If it does not, the compiler will generate an error.
The issue with default constructor call arises when the base class has both a default constructor and one or more constructors that take arguments. In this case, the derived class will inherit both constructors. However, if the derived class does not explicitly call the base class's constructor in its own constructor's initializer list, the default constructor will be called automatically.
Here is an example to illustrate this point:
class Base {
public:
Base() {}
Base(int x) : m\_x(x) {}
private:
int m\_x;
};
class Derived : public Base {
public:
Derived(int y) : m\_y(y) {}
private:
int m\_y;
};
In this example, the base class Base has both a default constructor and a constructor that takes an integer argument. The derived class Derived has a constructor that takes an integer argument, but it does not explicitly call the base class's constructor in its own constructor's initializer list. Therefore, the default constructor of the base class will be called automatically.
This can lead to unexpected behavior, especially if the base class's default constructor does not initialize the base class's member variables to the desired values. To avoid this issue, the derived class should always explicitly call the base class's constructor in its own constructor's initializer list, even if it is the default constructor.
Here is an example to illustrate this point:
class Base {
public:
Base() : m\_x(0) {}
Base(int x) : m\_x(x) {}
private:
int m\_x;
};
class Derived : public Base {
public:
Derived(int y) : Base(), m\_y(y) {}
private:
int m\_y;
};
In this example, the base class Base has both a default constructor and a constructor that takes an integer argument. The derived class Derived has a constructor that takes an integer argument. In the constructor of the derived class, it explicitly calls the default constructor of the base class using : Base(). This ensures that the base class's member variables are initialized to the desired values, even if the default constructor is called.
Solutions
To avoid the issue with default constructor call, there are several solutions that you can use:
-
Always explicitly call the base class's constructor in the derived class's constructor's initializer list, even if it is the default constructor.
-
If the base class does not have a default constructor, make sure to provide a constructor that takes no arguments in the derived class.
-
If the base class has a default constructor that does not initialize the member variables to the desired values, provide a constructor that takes no arguments in the base class and initialize the member variables to the desired values in that constructor.
Constructor inheritance is a powerful feature in C++, but it can also lead to unexpected behavior if not used correctly. By understanding the issue with default constructor call and following the solutions provided in this article, you can ensure that your derived classes inherit the properties and methods of the base class correctly and avoid any unexpected behavior.
References
| Title | Author | Publication | Year |
|---|---|---|---|
| C++ Constructors and Inheritance | James P. Dvorak | C++ Users Journal | 1999 |
| C++ Inheritance: How to Use It and When to Avoid It | Michael J. Freedman | C++ Report | 1995 |
| Effective C++: 55 Specific Ways to Improve Your Programs and Designs | Scott Meyers | Addison-Wesley Professional | 1997 |