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

Initialization Order of Non-Static Data Members in C

When creating a new instance of a class with non-static data members, the order in which they are initialized is crucial for proper object construction.

In this scenario, let's consider the following code:

class A {};
class B {};
class X
{
    A a;
    B b;
};

The question arises: when the constructor of class X is invoked, which constructor (A or B) is called first? Does their position within the class definition determine the order?

According to the C Standard, section 12.6.2, the initialization order is as follows:

• First, virtual base classes are initialized (not applicable in this case).
• Direct base classes are initialized in declaration order (also not applicable here).
• Non-static data members are initialized in the order they appear in the class definition.
• Finally, the constructor body is executed.

Therefore, in the provided code, the non-static data members a and b are initialized in the order they are declared, which is a first followed by b. The constructor of A will be called before the constructor of B.

In summary, the order of initialization for non-static data members is determined by their declaration order within the class definition, not by their placement within the constructor body.

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