C++ <vector> - vector() Function

The C++ vector::vector function is used to construct a vector object, initializing its contents depending on the version of constructor used:

Syntax

//default version - construct an empty 
//container with no elements
explicit vector (const allocator_type& alloc = allocator_type());

//fill version - construct a container 
//with n elements, each equal to val
explicit vector (size_type n, const value_type& val = value_type(),
                 const allocator_type& alloc = allocator_type());

//range version - Constructs a container with 
//elements as the range [first,last)
template <class InputIterator>
         vector (InputIterator first, InputIterator last,
                 const allocator_type& alloc = allocator_type());

//copy version - copies all elements
//of x into the container	
vector (const vector& x);

//default version - construct an empty 
//container with no elements
explicit vector (const allocator_type& alloc = allocator_type());

//fill version - construct a container 
//with n elements, each equal to val	
explicit vector (size_type n);
vector (size_type n, const value_type& val,
        const allocator_type& alloc = allocator_type());

//range version - Constructs a container with 
//elements as the range [first,last)
template <class InputIterator>
  vector (InputIterator first, InputIterator last,
          const allocator_type& alloc = allocator_type());

//copy version - copies all elements
//of x into the container	
vector (const vector& x);
vector (const vector& x, const allocator_type& alloc);

//move version - moves elements of x
//into the container	
vector (vector&& x);
vector (vector&& x, const allocator_type& alloc);

//initializer list version - copies all 
//elements of il into the container	
vector (initializer_list<value_type> il,
       const allocator_type& alloc = allocator_type());

//default version - construct an empty 
//container with no elements
vector();
explicit vector (const allocator_type& alloc);

//fill version - construct a container 
//with n elements, each equal to val
explicit vector (size_type n, const allocator_type& alloc = allocator_type());
vector (size_type n, const value_type& val,
        const allocator_type& alloc = allocator_type());

//range version - Constructs a container with 
//elements as the range [first,last)
template <class InputIterator>
  vector (InputIterator first, InputIterator last,
          const allocator_type& alloc = allocator_type());

//copy version - copies all elements
//of x into the container	
vector (const vector& x);
vector (const vector& x, const allocator_type& alloc);

//move version - moves elements of x
//into the container	
vector (vector&& x);
vector (vector&& x, const allocator_type& alloc);

//initializer list version - copies all 
//elements of il into the container		
vector (initializer_list<value_type> il,
       const allocator_type& alloc = allocator_type());

Parameters

`alloc`	Specify the Allocator object. The container keeps and uses an internal copy of this allocator.
`n`	Specify the number of elements in the container at construction.
`val`	Specify the value to fill the container with.
`first`	Specify initial position of the input iterator of the range. The range used is [first,last).
`last`	Specify final position of the input iterator of the range. The range used is [first,last).
`x`	Specify a vector object of same type.
`il`	Specify an initializer_list object.

Return Value

Constructor never returns value.

Time Complexity

Constant i.e, Θ(1), for default version and move version.
For all the other cases, Linear i.e, Θ(n).

Example: using default and fill version

In the example below, the vector::vector function is used to construct a vector object.

#include <iostream>
#include <vector>
using namespace std;
 
int main (){
  //default version - construct an empty vector
  vector<int> vec1;
  vector<int>::iterator it;

  //fill version - construct a vector
  //with 5 elements each equal to 100
  vector<int> vec2(5, 100);

  cout<<"vec1 contains: ";
  for(it = vec1.begin(); it != vec1.end(); ++it)
    cout<<*it<<" ";

  cout<<"\nvec2 contains: ";
  for(it = vec2.begin(); it != vec2.end(); ++it)
    cout<<*it<<" ";

  return 0;
}

The output of the above code will be:

vec1 contains: 
vec2 contains: 100 100 100 100 100

Example: using range and copy version

A vector can also be constructed using range or copy version. Consider the following example:

#include <iostream>
#include <vector>
using namespace std;
 
int main (){
  vector<int> vec1{100, 200, 300};
  vector<int>::iterator it;

  //range version - construct vec2 using range
  vector<int> vec2(vec1.begin(), vec1.end());

  //copy version - construct vec3 from vec1
  vector<int> vec3(vec1); 

  cout<<"vec2 contains: ";
  for(it = vec2.begin(); it != vec2.end(); ++it)
    cout<<*it<<" ";

  cout<<"\nvec3 contains: ";
  for(it = vec3.begin(); it != vec3.end(); ++it)
    cout<<*it<<" ";

  return 0;
}

The output of the above code will be:

vec2 contains: 100 200 300
vec3 contains: 100 200 300

Example: using move version

Using the move version of vector, the content of one vector can be moved to another vector. Consider the following example:

#include <iostream>
#include <vector>
using namespace std;
 
int main (){
  vector<int> vec1{10, 20, 30, 40, 50};
  vector<int>::iterator it;

  cout<<"vec1 contains: ";
  for(it = vec1.begin(); it != vec1.end(); ++it)
    cout<<*it<<" ";

  //moving all content of vec1 into vec2
  vector<int> vec2(move(vec1));

  cout<<"\nvec1 contains: ";
  for(it = vec1.begin(); it != vec1.end(); ++it)
    cout<<*it<<" ";

  cout<<"\nvec2 contains: ";
  for(it = vec2.begin(); it != vec2.end(); ++it)
    cout<<*it<<" ";

  return 0;
}

The output of the above code will be:

vec1 contains: 10 20 30 40 50 
vec1 contains: 
vec2 contains: 10 20 30 40 50

Example: using initializer list version

The initializer list can also be used to assign values into a vector container. Consider the example below:

#include <iostream>
#include <vector>
using namespace std;
 
int main (){
  //creating initializer list
  initializer_list<int> ilist = {15, 30, 45, 60, 75};

  //initializer list version - copies all 
  //elements of ilist into the container	
  vector<int> vec(ilist);
  vector<int>::iterator it;

  cout<<"vec contains: ";
  for(it = vec.begin(); it != vec.end(); ++it)
    cout<<*it<<" ";

  return 0;
}

The output of the above code will be:

vec contains: 15 30 45 60 75

❮ C++ <vector> Library