Is there a difference between copy initialization and direct initialization?

Question

Suppose I have this function:

void my_test()
{
    A a1 = A_factory_func();
    A a2(A_factory_func());

    double b1 = 0.5;
    double b2(0.5);

    A c1;
         


        

Answer 1

A lot of these cases are subject to an object's implementation so it's hard to give you a concrete answer.

Consider the case

A a = 5;
A a(5);

In this case assuming a proper assignment operator & initializing constructor which accept a single integer argument, how I implement said methods affects the behavior of each line. It is common practice however for one of those to call the other in the implementation as to eliminate duplicate code (although in a case as simple as this there would be no real purpose.)

Edit: As mentioned in other responses, the first line will in fact call the copy constructor. Consider the comments relating to the assignment operator as behavior pertaining to a stand alone assignment.

That said, how the compiler optimizes the code will then have it's own impact. If I have the initializing constructor calling the "=" operator - if the compiler makes no optimizations, the top line would then perform 2 jumps as opposed to one in the bottom line.

Now, for the most common situations, your compiler will optimize through these cases and eliminate this type of inefficiencies. So effectively all the different situations you describe will turn out the same. If you want to see exactly what is being done, you can look at the object code or an assembly output of your compiler.