emplace_back 示例

std::vector::emplace_back，执行效率优于push_back。

template <class... Args>
  void emplace_back (Args&&... args);
Construct and insert element at the end
Inserts a new element at the end of the vector, right after its current last element. This new element is constructed in place using args as the arguments for its constructor.

This effectively increases the container size by one, which causes an automatic reallocation of the allocated storage space if -and only if- the new vector size surpasses the current vector capacity.

The element is constructed in-place by calling allocator_traits::construct with args forwarded.

A similar member function exists, push_back, which either copies or moves an existing object into the container.

// vector::emplace_back
#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> myvector = {10,20,30};

  myvector.emplace_back (100);
  myvector.emplace_back (200);

  std::cout << "myvector contains:";
  for (auto& x: myvector)
    std::cout << ' ' << x;
  std::cout << '\n';

  return 0;
}

Output:
myvector contains: 10 20 30 100 200