Function in C++ returns by value or by reference?

Question

When a function (callee) returns a quantity to the caller function, is it returned by value or by reference?

The thing is I have written a function which builds a ve

Answer 1

C++ can return either by reference or by value. If you want to return a reference, you must specify that as part of the return type:

std::vector my_func(); // returns value
std::vector& my_func(); // returns reference
std::vector const& my_func(); // returns constant reference

All local (stack) variables created inside of a function are destroyed when the function returns. That means you should absolutely not return locals by reference or const reference (or pointers to them). If you return the vector by value it may be copied before the local is destroyed, which could be costly. (Certain types of optimizations called "return value optimization" can sometimes remove the copy, but that's out of the scope of this question. It's not always easy to tell whether the optimization will happen on a particular piece of code.)

If you want to "create" a large vector inside of a function and then return it without copying, the easiest way is to pass the vector in to the function as a reference parameter:

void fill_vector(std::vector &vec) {
    // fill "vec" and don't return anything...
}

Also note that in the recently ratified new version of the C++ standard (known as C++0x or C++11) returning a local vector by value from a function will not actually copy the vector, it will be efficiently moved into its new location. The code that does this looks identical to code from previous versions of C++ which could be forced to copy the vector. Check with your compiler to see whether it supports "move semantics" (the portion of the C++11 standard that makes this possible).