Why couldn't push_back be overloaded to do the job of emplace_back?

45
votes

Firstly, I'm aware of this question, but I don't believe I'm asking the same thing.

I know what std::vector<T>::emplace_back does - and I understand why I would use it over push_back(). It uses variadic templates allowing me to forward multiple arguments to the constructor of a new element.

But what I don't understand is why the C++ standard committee decided there was a need for a new member function. Why couldn't they simply extend the functionality of push_back(). As far as I can see, push_back could be overloaded in C++11 to be:

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

This would not break backwards compatibility, while allowing you to pass N arguments, including arguments that would invoke a normal rvalue or copy constructor. In fact, the GCC C++11 implementation of push_back() simply calls emplace_back anyway:

  void push_back(value_type&& __x)
  { 
    emplace_back(std::move(__x)); 
  }

So, the way I see it, there is no need for emplace_back(). All they needed to add was an overload for push_back() which accepts variadic arguments, and forwards the arguments to the element constructor.

Am I wrong here? Is there some reason that an entirely new function was needed here?

2
c++c++11
this looks similar: stackoverflow.com/questions/4303513/push-back-vs-emplace-back – moka
Good question. @moka he knows the difference between what the two functions do. He even linked to that particular question. – user802003
They have different meanings, it's all explained in the question you mentioned you read. Compare v.emplace_back(123) and v.push_back(123) for example for vector<SomeType> v; with implicit conversion from int. – Gene Bushuyev
@Gene: Okay, I've compared them, and I don't see what the difference would be if push_back was simply modified to do what emplace_back does. Would you mind expounding upon it here? – Benjamin Lindley
That's pretty standard practice for library development. You don't change your interface you make a new one. Then allow users to implement the old API in terms of the new API at their own pace while the code never becomes broken. – AJG85

2 Answers

42
votes

If T has an explicit conversion constructor, there is different behavior between emplace_back and push_back.

struct X
{
    int val;
    X() :val() {}
    explicit X(int v) :val(v) {}
};

int main()
{
    std::vector<X> v;
    v.push_back(123);    // this fails
    v.emplace_back(123); // this is okay
}

Making the change you suggest would mean that push_back would be legal in that instance, and I suppose that was not desired behavior. I don't know if this is the reason, but it's the only thing I can come up with.

0
votes

Here's another example.

Honestly, the two are semantically so different, that their similar behavior should be regarded as a mere coincidence (due to the fact that C++ has "copy constructors" with a particular syntax).

You should really not use emplace_back unless you want in-place-construction semantics.
It's rare that you'd need such a thing. Generally push_back is what you really, semantically want.

#include <vector>

struct X { X(struct Y const &); };
struct Y { Y(int const &); operator X(); };

int main()
{
    std::vector<X> v;
    v.   push_back(Y(123));  // Calls Y::operator X() and Y::Y(int const &)
    v.emplace_back(Y(123));  // Calls X::X(Y const &) and Y::Y(int const &)
}