Efficient way to return a std::vector in c++

c++ return-value stdvector
Morten picture Morten · Mar 29, 2013 · Viewed 146.4k times · Source

How much data is copied, when returning a std::vector in a function and how big an optimization will it be to place the std::vector in free-store (on the heap) and return a pointer instead i.e. is:

std::vector *f()
{
  std::vector *result = new std::vector();
  /*
    Insert elements into result
  */
  return result;
}

more efficient than:

std::vector f()
{
  std::vector result;
  /*
    Insert elements into result
  */
  return result;
}

?

Answer

Nawaz picture Nawaz · Mar 29, 2013

In C++11, this is the preferred way:

std::vector<X> f();

That is, return by value.

With C++11, std::vector has move-semantics, which means the local vector declared in your function will be moved on return and in some cases even the move can be elided by the compiler.

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