How is the initialization and destruction order of member objects in C aggregates determined?

Member Initialization and Destruction Order in C

In C , the order in which member objects are initialized and destructed is crucial for understanding the behavior of complex data structures. Let's consider the following program:

#include <iostream>
using namespace std;

struct A
{
    A() { cout <p>This program creates an instance of an aggregate struct that contains three other structs, A, B, and C. When we execute this program, the following output is produced:</p>
<pre class="brush:php;toolbar:false">A::A
B::B
C::C
C::~
B::~
A::~

This output raises the question: is this order of construction and destruction guaranteed by the C standard?

The answer is yes. According to the C standard (12.6.2), members of an aggregate struct are initialized and destructed in the following order:

• Initialization Order:

  • First, virtual base classes are initialized in the order they appear in the derived class declaration.
  • Then, direct base classes are initialized in the order they appear in the base class list.
  • Finally, non-static data members are initialized in the order they are declared in the class definition.

• Destruction Order:

  • Non-static data members are destructed in the reverse order they were initialized.
  • Direct base classes are destructed in the reverse order they were initialized.
  • Virtual base classes are destructed in the opposite order they were initialized.

Therefore, for the given program, the order of member construction and destruction is guaranteed to be:

1. Initialization: A::A, B::B, C::C
2. Destruction: C::~, B::~, A::~

This guarantees consistent behavior when creating and destroying complex data structures in C .

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