What is the purpose of the colon operator (':') in C constructors, and how does it differ from explicit member variable assignments?

Initializer Lists in C Constructors

The colon operator (":") used in the provided C code is part of a member initializer list, which is a way to initialize class member variables within a constructor. Contrary to the suggestion in the question, it is not equivalent to explicit individual assignments.

In this example:

class MyClass {
public:

    MyClass() : m_classID(-1), m_userdata(0) { 
    }

    int m_classID;
    void *m_userdata;
};

The constructor MyClass() is a default constructor that initializes two member variables, m_classID and m_userdata, to the values -1 and 0 respectively. This is done using the member initializer list, which follows the constructor's parameter list and precedes the constructor body.

The member initializer list initializes class member variables in the order they are declared in the class. Each initialization is separated by a comma. The values specified in the initializer list are immediately assigned to the corresponding member variables when the constructor is called.

In contrast, performing individual assignments within the constructor body, as suggested in the question, would result in explicit changes to values that have already been initialized. This is different from the initialization performed by the member initializer list.

Therefore, the member initializer list provides a convenient way to initialize class member variables within a constructor, and it differs from explicit individual assignments.

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