Why Isn't the Copy Constructor Called in `A a = A(5);`?

Understanding Why the Copy Constructor Is Not Called

In the provided code snippet, the constructor of class A is not invoked for the left-hand side object, despite the assumption that it should be. This behavior is not due to compiler optimization but rather an established feature of C .

According to the C standard (§12.8.15), the statement T = x; is equivalent to T(x);. Thus, in the code snippet:

A a = A(5);

The system first constructs A(5) and then assigns it to a. The copy constructor is not invoked because the assignment operator (=) is used.

To force the compiler to construct the left-hand side object defaultly, one can write the following code:

A a;
// a is now a fully constructed object
a = A(5);

In this scenario, a is default-constructed first, and the copy constructor is then invoked due to the object's full construction.

Therefore, it is crucial to be aware of this behavior and its potential implications in C programming.

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