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 << "A::A" << endl; }
    ~A() { cout << "A::~" << endl; }
};

struct B
{
    B() { cout << "B::B" << endl; }
    ~B() { cout << "B::~" << endl; }
};

struct C
{
    C() { cout << "C::C" << endl; }
    ~C() { cout << "C::~" << endl; }
};

struct Aggregate
{
    A a;
    B b;
    C c;
};

int main()
{
    Aggregate a;
    return 0;
}

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:

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 .

The above is the detailed content of How is the initialization and destruction order of member objects in C aggregates determined?. For more information, please follow other related articles on the PHP Chinese website!