difference between push_back and emplace_back with C++ 11

Publish Date: 2019-04-22

Word Count: 380

Read Times: 2 Min

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Guide

case1

#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"; }
  ~A() { std::cout << "~A ()\n"; }

private:
  int x;
};

void test_emplace_back_1()
{
    // 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
  {
    std::vector<A> a;
    std::cout << "call emplace_back:\n";
    a.emplace_back(0); 
    // (1) direct object creation inside vector
  }

  {
    std::vector<A> a;
    std::cout << "call push_back:\n";
    a.push_back(1);
    // (1) create temp object and 
    // (2) then move copy to vector and 
    // (3) free temp object
  }
}
/*
call emplace_back:
A (x_arg)
~A ()
call push_back:
A (x_arg)
A (A &&)
~A ()
~A ()
 */

see kezunlin

image from c-difference-between-emplace_back-and-push_back-function

case2

void test_emplace_back_2()
{
    // emplace_back and push_back for `A(0)`, it's same.
    // A (int x_arg) is called first and 
    // move A (A &&rhs) is called afterwards
  {
    std::vector<A> a;
    std::cout << "call emplace_back:\n";
    a.emplace_back(A(0)); 
    // (1) create temp object and 
    // (2) then move copy to vector and 
    // (3) free temp object
  }

  {
    std::vector<A> a;
    std::cout << "call push_back:\n";
    a.push_back(A(1));
    // (1) create temp object and 
    // (2) then move copy to vector and 
    // (3) free temp object
  }
}

/*
call emplace_back:
A (x_arg)
A (A &&)
~A ()
~A ()
call push_back:
A (x_arg)
A (A &&)
~A ()
~A ()
 */

case 3

void test_emplace_back_3()
{
    // emplace_back and push_back for `A obj(0)`, it's same.
    // A (int x_arg) is called first and 
    // copy constructor A (A &) is called afterwards
  {
    std::vector<A> a;
    std::cout << "call emplace_back:\n";
    A obj(0);
    a.emplace_back(obj); 
     // copy constructor to vector
  }

  {
    std::vector<A> a;
    std::cout << "call push_back:\n";
    A obj(1);
    a.push_back(obj);
     // copy constructor to vector
  }
}
/*
call emplace_back:
A (x_arg)
A (A &)
~A ()
~A ()
call push_back:
A (x_arg)
A (A &)
~A ()
~A ()
 */

extract subvector

vector<int>::const_iterator first = myVec.begin() + 10;
vector<int>::const_iterator last = myVec.begin() + 15;
vector<int> newVec(first, last); // [10,15)