When Does C Skip the Copy Constructor?

Copy Constructor Optimization in C

In C , under specific circumstances, the copy constructor may not be invoked, prompting questions about compiler optimization or the language's undocumented features.

Consider the following code:

class A
{
public:
    A(int value) : value_(value)
    {
        cout << "Regular constructor" << endl;
    }

    A(const A& other) : value_(other.value_)
    {
        cout << "Copy constructor" << endl;
    }

private:
    int value_;
};

int main()
{
    A a = A(5);
}

One would expect the output to include both "Regular constructor" and "Copy constructor" messages. However, in this case, the copy constructor is never called.

This behavior is neither a compiler optimization nor an undocumented feature of C . Instead, it is explicitly specified in the C standard (§12.8.15) that assignments like T = x; can be interpreted as T(x);, effectively eliminating the inner T when there is no need for a copy.

In this particular case, the compiler recognizes that constructing an A object and then copying it over is redundant and therefore omits the copy constructor call.

To enforce the copy constructor's invocation, one can explicitly create the first A object:

A a; // Construct an empty A object
a = A(5); // Copy-initialize it with another A object

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