push_back vs emplace_back

Solution 1:

In addition to what visitor said :

The function void emplace_back(Type&& _Val) provided by MSCV10 is non conforming and redundant, because as you noted it is strictly equivalent to push_back(Type&& _Val).

But the real C++0x form of emplace_back is really useful: void emplace_back(Args&&...);

Instead of taking a value_type it takes a variadic list of arguments, so that means that you can now perfectly forward the arguments and construct directly an object into a container without a temporary at all.

That's useful because no matter how much cleverness RVO and move semantic bring to the table there is still complicated cases where a push_back is likely to make unnecessary copies (or move). For example, with the traditional insert() function of a std::map, you have to create a temporary, which will then be copied into a std::pair<Key, Value>, which will then be copied into the map :

std::map<int, Complicated> m;
int anInt = 4;
double aDouble = 5.0;
std::string aString = "C++";

// cross your finger so that the optimizer is really good
m.insert(std::make_pair(4, Complicated(anInt, aDouble, aString))); 

// should be easier for the optimizer
m.emplace(4, anInt, aDouble, aString);

So why didn't they implement the right version of emplace_back in MSVC? Actually, it bugged me too a while ago, so I asked the same question on the Visual C++ blog. Here is the answer from Stephan T Lavavej, the official maintainer of the Visual C++ standard library implementation at Microsoft.

Q: Are beta 2 emplace functions just some kind of placeholder right now?

A: As you may know, variadic templates aren't implemented in VC10. We simulate them with preprocessor machinery for things like make_shared<T>(), tuple, and the new things in <functional>. This preprocessor machinery is relatively difficult to use and maintain. Also, it significantly affects compilation speed, as we have to repeatedly include subheaders. Due to a combination of our time constraints and compilation speed concerns, we haven't simulated variadic templates in our emplace functions.

When variadic templates are implemented in the compiler, you can expect that we'll take advantage of them in the libraries, including in our emplace functions. We take conformance very seriously, but unfortunately, we can't do everything all at once.

It's an understandable decision. Everyone who tried just once to emulate variadic template with preprocessor horrible tricks knows how disgusting this stuff gets.

Solution 2:

emplace_back shouldn't take an argument of type vector::value_type, but instead variadic arguments that are forwarded to the constructor of the appended item.

template <class... Args> void emplace_back(Args&&... args); 

It is possible to pass a value_type which will be forwarded to the copy constructor.

Because it forwards the arguments, this means that if you don't have rvalue, this still means that the container will store a "copied" copy, not a moved copy.

 std::vector<std::string> vec;
 vec.emplace_back(std::string("Hello")); // moves
 std::string s;
 vec.emplace_back(s); //copies

But the above should be identical to what push_back does. It is probably rather meant for use cases like:

 std::vector<std::pair<std::string, std::string> > vec;
 vec.emplace_back(std::string("Hello"), std::string("world")); 
 // should end up invoking this constructor:
 //template<class U, class V> pair(U&& x, V&& y);
 //without making any copies of the strings

Solution 3:

Optimization for emplace_back can be demonstrated in next example.

For emplace_back constructor A (int x_arg) will be called. And for push_back A (int x_arg) is called first and move A (A &&rhs) is called afterwards.

Of course, the constructor has to be marked as explicit, but for current example is good to remove explicitness.

#include <iostream>
#include <vector>
class A
{
public:
  A (int x_arg) : x (x_arg) { std::cout << "A (x_arg)\n"; }
  A () { x = 0; std::cout << "A ()\n"; }
  A (const A &rhs) noexcept { x = rhs.x; std::cout << "A (A &)\n"; }
  A (A &&rhs) noexcept { x = rhs.x; std::cout << "A (A &&)\n"; }

private:
  int x;
};

int main ()
{
  {
    std::vector<A> a;
    std::cout << "call emplace_back:\n";
    a.emplace_back (0);
  }
  {
    std::vector<A> a;
    std::cout << "call push_back:\n";
    a.push_back (1);
  }
  return 0;
}

output:

call emplace_back:
A (x_arg)

call push_back:
A (x_arg)
A (A &&)