C++ Inserting Elements


Example

Appending an element at the end of a vector (by copying/moving):

struct Point {
  double x, y;
  Point(double x, double y) : x(x), y(y) {}
};
std::vector<Point> v;
Point p(10.0, 2.0);
v.push_back(p);  // p is copied into the vector.
C++11

Appending an element at the end of a vector by constructing the element in place:

std::vector<Point> v;
v.emplace_back(10.0, 2.0); // The arguments are passed to the constructor of the
                           // given type (here Point). The object is constructed
                           // in the vector, avoiding a copy.

Note that std::vector does not have a push_front() member function due to performance reasons. Adding an element at the beginning causes all existing elements in the vector to be moved. If you want to frequently insert elements at the beginning of your container, then you might want to use std::list or std::deque instead.


Inserting an element at any position of a vector:

std::vector<int> v{ 1, 2, 3 };
v.insert(v.begin(), 9);          // v now contains {9, 1, 2, 3}
C++11

Inserting an element at any position of a vector by constructing the element in place:

std::vector<int> v{ 1, 2, 3 };
v.emplace(v.begin()+1, 9);     // v now contains {1, 9, 2, 3}

Inserting another vector at any position of the vector:

std::vector<int> v(4);      // contains: 0, 0, 0, 0
std::vector<int> v2(2, 10); // contains: 10, 10
v.insert(v.begin()+2, v2.begin(), v2.end()); // contains: 0, 0, 10, 10, 0, 0

Inserting an array at any position of a vector:

std::vector<int> v(4); // contains: 0, 0, 0, 0
int a [] = {1, 2, 3}; // contains: 1, 2, 3
v.insert(v.begin()+1, a, a+sizeof(a)/sizeof(a[0])); // contains: 0, 1, 2, 3, 0, 0, 0

Use reserve() before inserting multiple elements if resulting vector size is known beforehand to avoid multiple reallocations (see vector size and capacity):

std::vector<int> v;
v.reserve(100);
for(int i = 0; i < 100; ++i)
    v.emplace_back(i);

Be sure to not make the mistake of calling resize() in this case, or you will inadvertently create a vector with 200 elements where only the latter one hundred will have the value you intended.