c++ vector push_back() vs emplace_back

std::vector::push_back()

Appends the given element value to the end of the container.

• The new element is initialized as a copy of value.
• value is moved into the new element.

If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated. (Cite from: cppreference.com)

Example

Firstly, if we have a class String, then we can use this class to test the push_back() functions.

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#include <cstring>
#include <vector>
class  {
 public:
  string() : buf(new char[1]) {
    std::cout << "default" << std::endl;
    buf[0] = '