CODEWITHSUNDEEP
c++destructor
Sorin
Sorin

Reputation: 910

Why is my destructor called twice?

Suppose I hace a class Student with the method:

Student Student::method(Student x)
{
    //nothing important
    return x;
}

The copy constructor is called twice, once when the object x is send as a parameter and second when x is returned from the function.

Why and when is the destructor for class Student called twice when I call this method? The call is like this: a = b.method(c), where a, b and c are Student objects.

Upvotes: 1

Views: 1197

Answers (3)

Joseph Mansfield
Joseph Mansfield

Reputation: 110748

For your example, a = b.method(c);, there are three copies that may take place, save for copy elision. The first is when the c object is copied into the function parameter x. The second is when the x object is returned from the function. The third is when the return value is copied into the a object. The first two involve the copy constructor and the last involves the copy assignment operator, unless you change it to Student a = b.method(c);, in which case they all use the copy constructor.

a, b, and c will all be destroyed at the end of their scope. The object x will be destroyed at the end of the method function. The return value of the function will be destroyed at the end of the full expression that contains it - that is, once a = b.method(c); has finished.

However, not all of these copies must occur - the compiler is allowed to elide or omit the copy/move construction of a class under certain situations. The first copy into the function parameter will occur. The second copy out of the function will be treated as a move first, before attempting to copy it. This copy or move may be elided. The final copy, from temporary return value to a, will occur if you're using copy assignment, but may be elided if you use the copy constructor (as in Student a = b.method(c);).

Upvotes: 3

Mankarse
Mankarse

Reputation: 40633

The destructor for x is called when the function returns (after x has been copied in to the return value).

The destructor for the return value is called at the end of the full-expression containing the function call (unless the return value has its lifetime extended by being assigned to a reference).

Every object with automatic storage duration that is constructed will automatically be destructed (usually in reverse order of construction). You construct two objects (x and the return value) and so there are two destructor calls.

Upvotes: 1

Puppy
Puppy

Reputation: 147018

If two Student objects are constructed, they must be destructed. The copies into the parameter and out of the return value need destructing.

Upvotes: 1

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