When is a const reference better than pass-by-value in C++11?

Question

I have some pre-C++11 code in which I use const references to pass large parameters like vector\'s a lot. An example is as follows:

Answer 1

The general rule of thumb for passing by value is when you would end up making a copy anyway. That is to say that rather than doing this:

void f(const std::vector& x) {
    std::vector y(x);
    // stuff
}

where you first pass a const-ref and then copy it, you should do this instead:

void f(std::vector x) {
    // work with x instead
}

This has been partially true in C++03, and has become more useful with move semantics, as the copy may be replaced by a move in the pass-by-val case when the function is called with an rvalue.

Otherwise, when all you want to do is read the data, passing by const reference is still the preferred, efficient way.