What is the order of member constructor and destructor calls in C ?

Order of Member Constructor and Destructor Calls: A C Specification

In C , the order of initialization and destruction of class members is crucial for managing object lifetimes and memory allocation efficiently. This article sheds light on the specifics of this process, as defined by the C language standard.

Consider the following code snippet:

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;
}

When this code executes, the constructor and destructor calls will occur in a specific order.

Member Initialization Order:

As per C 14 standard section 12.6.2:

• Static data members are initialized first, in the order of declaration.
• Non-static data members are initialized next, also in the order of declaration. Here, member a is initialized first, followed by b and then c.

Therefore, the expected output for member initialization is:

A::A
B::B
C::C

Member Destruction Order:

The order of member destruction is the reverse of initialization:

• Non-static data members are destroyed in the reverse order of declaration.
• Static data members are destroyed last.

The expected output for member destruction is thus:

C::~
B::~
A::~

This confirms that C guarantees the order of member construction and destruction as specified in the standard. Members are initialized in declaration order and destroyed in reverse declaration order, ensuring proper memory management and object lifecycle. Understanding these rules is essential for writing efficient and correct C code.

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