Returning an STL vector from a function - copy cost

Question

When you return an stl vector from a function:

vector getLargeArray() {  ...  }

Is the return going to be an expensive copy operatio

Answer 1

You are very likely to get return value optimization, depending on the structure of the function body. In C++11 you could also benefit from move semantics. Returning by value certainly has cleaner semantics, and I would see it as the preferred option, unless profiling proves it to be costly. There is a good related article here.

Here is a small example with a verbose dummy class, compiled with no optimization or C++11 support, using an old-ish version of GCC (4.3.4):

#include 
#include 
struct Foo
{
  Foo() { std::cout << "Foo()\n"; }
  Foo(const Foo&) { std::cout << "Foo copy\n"; }
  Foo& operator=(const Foo&) { 
    std::cout << "Foo assignment\n"; 
    return *this;
  }
};

std::vector makeFoos()
{
  std::vector tmp;
  tmp.push_back(Foo());
  std::cout << "returning\n";
  return tmp;
}

int main()
{
  std::vector foos = makeFoos();
}

The result on my platform is that all copying happens before the function return. If I compile with C++11 support, then the push_back results in a move copy rather than a C++03 copy construction.