How to Guarantee Initialization Order in C Member Initializer Lists?

Initialization Order in Member Initializer Lists

In C , constructors often initialize member variables using a member initializer list. However, the order in which these variables are initialized can be confusing.

Consider the following constructor:

class A {
public:
    OtherClass a_;
    AnotherClass b_;

    A(OtherClass o, string x, int y)
    : a_(o)
    , b_(a_, x, y) {}
};

Problem:

Originally, it was assumed that the variables were initialized in the order listed in the initializer list: a_ first, then b_. However, an issue arose when it turned out that the variables were being initialized in reverse order.

Question:

How can the order of initialization be controlled to ensure that a_ is initialized before b_?

Answer:

The order of initialization depends on the order of data member declarations in the class. In this example, a_ is declared before b_, so it will be initialized first.

Therefore, to ensure the desired order of initialization, it is crucial to declare the data members in the correct order within the class definition.

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