C Constructors: What Does the Colon After the Constructor Mean?

C : Understanding the Colon After a Constructor

In C , a colon following a constructor indicates the use of a member initializer list. This list serves two primary purposes:

• Calling base class constructors for inherited classes
• Initializing data members before the constructor body execution

Calling Base Class Constructors

When a derived class is defined, the member initializer list can be used to invoke the constructor of the base class. This is achieved by specifying the name of the base class constructor followed by the appropriate arguments. For instance, in the example code you provided:

class newdemo : public demo {
public:
    newdemo(void) : demo(0, 0) {
        // Constructor body
    }
};

In this case, the : demo(0, 0) initializes the base class demo with the parameters 0 and 0.

Initializing Data Members

The member initializer list can also be employed to pre-initialize data members before the constructor body executes. This is especially useful for const data members or reference data members. For const members, initializing them in the constructor body is not permitted, as their values cannot be modified once assigned.

class Demo {
public:
    Demo(int& val) : m_val(val) {}
private:
    const int& m_val;
};

In this example, the const data member m_val is initialized using the member initializer list. This is the only permitted location to assign a value to a const data member. Similarly, reference data members can only be initialized via the member initializer list.

Additional Benefits

Beyond the above uses, the member initializer list has become a common practice in C code. It improves code readability and serves as a clear indication of data member initialization.

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