c++ vector::emplace

转自：http://www.cplusplus.com/reference/vector/vector/emplace/
template <class... Args>
iterator emplace (const_iterator position, Args&&... args);

vector::emplace 

在position位置插入一个元素

转自：http://www.cplusplus.com/reference/vector/vector/push_back/

http://www.cplusplus.com/reference/vector/vector/push_back/

https://stackoverflow.com/questions/4303513/push-back-vs-emplace-back

Construct and insert element

The container is extended by inserting a new element at position. This new element is constructed in place using argsas the arguments for its construction.

This effectively increases the container size by one.

Because vectors use an array as their underlying storage, inserting elements in positions other than the vector endcauses the container to shift all the elements that were after position by one to their new positions. This is generally an inefficient operation compared to the one performed by other kinds of sequence containers (such as list or forward_list). See emplace_back for a member function that extends the container directly at the end.

在position位置插入一个元素后，原position后的元素都要一次后移一个位置

 

Return value

An iterator that points to the newly emplaced element. 返回指向新插入元素的迭代器

Member type iterator is a random access iterator type that points to an element.

 

Example

 

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

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

  auto it = myvector.emplace ( myvector.begin()+1, 100 );
  myvector.emplace ( it, 200 );
  myvector.emplace ( myvector.end(), 300 );

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

  return 0;
}

Outputs:

myvector contains: 10 200 100 20 30 300

 emplace_back vs push_back

std::vector::emplace_back

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

std::vector::push_back

 

void push_back (const value_type& val);
void push_back (value_type&& val);

emplace_back 传递的是参数列表， push_back传递的是value_type

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.

push_back()会调用一次构造函数+一次拷贝构造函数

emplace_back()只调用一次构造函数

#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;
}

输出：

 

One more in case of lists:

// constructs the elements in place.
emplace_back("element");

//It will create new object and then copy(or move) its value of arguments. push_back(explicitDataType{"element"});

---

emplace_back() vs push_back()

https://www.geeksforgeeks.org/emplace-vs-insert-c-stl/
When we use push_back(), we create an object and then insert it into the vector. With emplace_back(), the object is constructed in-place and saves an unnecessary copy. Please see emplace vs insert in C++ STL for details.

 

// C++ code to demonstrate difference between // emplace_back and insert_back #include<bits/stdc++.h> using namespace std;      int main() {     // declaring priority queue     vector<pair<char, int>> vect;              // using emplace() to insert pair in-place     vect.emplace_back('a', 24);              // Below line would not compile     // vect.push_back('b', 25);                  // using push_back() to insert     vect.push_back(make_pair('b', 25));                  // printing the vector     for (int i=0; i<vect.size(); i++)         cout << vect[i].first << " " << vect[i].second              << endl;         return 0; }

Output :

a 24
b 25

 

Output:

 a 24
 b 25