RVO and std :: vector

Question

I wrote the code below to verify that popular C ++ 03 compilers implement RVO whenever possible. (See my related question on RVO Return Value Optimization: ho can I avoid creating a copy of the huge STL containers. ).

As far as I understand, the short answer is no. But I'm not sure I understand.

I have the following five questions about the implementation of std :: vector clang gcc and VS9, read the code and the result below:

• Why does the C Fun1 compiler copy a vector twice?
• Why do both B and C compilers always build at least one unsolicited element?
• Why is this unsolicited item executed twice with the C Fun2 compiler?
• Why does space reservation provide another unsolicited element for compilers B and C?
• Is all this a standard match?
• Bonus question: does all this meet user expectations?

---

#include <iostream>
#include <vector>
#include <cassert>

#define SIZE (3) 

class NRVO{
public:
  NRVO() : name_(-1){}
  ~NRVO(){
    std::cout << "Destroy "<< name_ << "\n";
  }
  void set_name(){name_ = counter++;}
private:
  int name_;
  static int counter; 
};

int NRVO::counter = 0;

std::vector<NRVO> fun1(){
  std::vector<NRVO> vec(SIZE);  
  for(std::vector<NRVO>::iterator v_it = vec.begin();
      v_it != vec.end();
      ++v_it){
    v_it->set_name();
  }
  return vec;
}

void fun2(std::vector<NRVO>& vec){
  vec.clear();
  vec.resize(SIZE);  
  for(std::vector<NRVO>::iterator v_it = vec.begin();
      v_it != vec.end();
      ++v_it){
    v_it->set_name();
  }
  return;
}

int main(){
  {
    std::vector<NRVO> myNrvo1;
    std::cout << "Fun1\n";
    myNrvo1 = fun1();
    assert(myNrvo1.size()==SIZE);
  }
  {
    std::vector<NRVO> myNrvo2;
    std::cout << "Fun2\n";
    fun2(myNrvo2);
    assert(myNrvo2.size()==SIZE);
  }
  {
    std::vector<NRVO> myNrvo3;
    myNrvo3.reserve(SIZE);
    std::cout << "Fun3\n";
    fun2(myNrvo3);
    assert(myNrvo3.size()==SIZE);
  }
  return 0;
}

Output using the popular C ++ 11 compiler

Fun1
Destroy 0
Destroy 1
Destroy 2
Fun2
Destroy 3
Destroy 4
Destroy 5
Fun3 
Destroy 6
Destroy 7
Destroy 8

Output using the popular C ++ 03 compiler

Fun1
Destroy -1
Destroy 0
Destroy 1
Destroy 2
Destroy 0
Destroy 1
Destroy 2
Fun2
Destroy -1
Destroy 3
Destroy 4
Destroy 5
Fun3 
Destroy -1
Destroy -1
Destroy 6
Destroy 7
Destroy 8

Output using the popular C ++ 03 C compiler

Fun1
Destroy -1
Destroy 0
Destroy 1
Destroy 2
Destroy 0
Destroy 1
Destroy 2
Destroy 0
Destroy 1
Destroy 2
Fun2
Destroy -1
Destroy -1
Destroy 3
Destroy 4
Destroy 5
Fun3
Destroy -1
Destroy -1
Destroy -1
Destroy 6
Destroy 7
Destroy 8