is there any specific case where pass-by-value is preferred over pass-by-const-reference in C++?

Question

I read that they are conceptually equal. In practice, is there any occasion that

foo(T t) 

is preferred over

foo(const T&         


        

Answer 1

They're not conceptually equal at all...

The former creates a copy of the object inside the function. This implies that the value can be modified in the function safely. It also implies that a full copy of the object occurred, which can be a problem if the object is large.

The latter creates an alias to the object and states that it cannot be modified within the object. No copying occurs, but each access to the object inside the function will require a dereference. The compiler takes care of this for us, but it's still important to know.

The difference becomes very important if you have a type that is normally passed in registers. For example, integers, floating point numbers, and even 4-float vectors on some platforms. Performance concerns dictate that you want to object to stay in a register for as long as possible without writing itself back to memory, and pass by value makes this much more likely.

So for basic types (char, short, int, long, float, double), you should always prefer pass by value unless you specifically need to use a reference to store a value for use after the function exits. For full objects, generally prefer to pass by const reference.