What's the Initialization Order of Non-Static Data Members in C ?

Initialization Order of Non-Static Data Members

In this scenario, we have two non-static data members, a and b, declared within the class X. A common question arises: in what order are these data members initialized when the constructor for X is invoked?

To answer this question, we turn to section 12.6.2 of the C Standard, which outlines the initialization order of class members:

5 Initialization shall proceed in the following order:
-- First, and only for the constructor of the most derived class as described below, virtual base classes shall be initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes...
-- Then, direct base classes shall be initialized in declaration order as they appear in the base-specifier-list...
-- Then, nonstatic data members shall be initialized in the order they were declared in the class definition...
-- Finally, the body of the constructor is executed...

Based on this rule, the order of initialization of a and b is determined solely by their placement within the class definition. Since a precedes b, it will be initialized first. This holds true regardless of any explicitly specified order of member initialization within the constructor body.

Therefore, in this case, the constructor of A will be called before the constructor of B when the constructor of X is invoked.

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