public member function
<vector>

std::vector::assign

range (1)
template <class InputIterator>
  void assign (InputIterator first, InputIterator last);
fill (2)
void assign (size_type n, const value_type& val);
range (1)
template <class InputIterator>
  void assign (InputIterator first, InputIterator last);
fill (2)
void assign (size_type n, const value_type& val);
initializer list (3)
void assign (initializer_list<value_type> il);
Assign vector content
Assigns new contents to the vector, replacing its current contents, and modifying its size accordingly.

In the range version (1), the new contents are elements constructed from each of the elements in the range between first and last, in the same order.

In the fill version (2), the new contents are n elements, each initialized to a copy of val.

If a reallocation happens,the storage needed is allocated using the internal allocator.
In the range version (1), the new contents are elements constructed from each of the elements in the range between first and last, in the same order.

In the fill version (2), the new contents are n elements, each initialized to a copy of val.

In the initializer list version (3), the new contents are copies of the values passed as initializer list, in the same order.

The internal allocator is used (through its traits) to allocate and deallocate storage if a reallocation happens. It is also used to destroy all existing elements, and to construct the new ones.

Any elements held in the container before the call are destroyed and replaced by newly constructed elements (no assignments of elements take place).

This causes an automatic reallocation of the allocated storage space if -and only if- the new vector size surpasses the current vector capacity.

Parameters

first, last
Input iterators to the initial and final positions in a sequence. The range used is [first,last), which includes all the elements between first and last, including the element pointed by first but not the element pointed by last.
The function template argument InputIterator shall be an input iterator type that points to elements of a type from which value_type objects can be constructed.
n
New size for the container.
Member type size_type is an unsigned integral type.
val
Value to fill the container with. Each of the n elements in the container will be initialized to a copy of this value.
Member type value_type is the type of the elements in the container, defined in vector as an alias of its first template parameter (T).
il
An initializer_list object. The compiler will automatically construct such objects from initializer list declarators.
Member type value_type is the type of the elements in the container, defined in vector as an alias of its first template parameter (T).

Return value

none

Example

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// vector assign
#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> first;
  std::vector<int> second;
  std::vector<int> third;

  first.assign (7,100);             // 7 ints with a value of 100

  std::vector<int>::iterator it;
  it=first.begin()+1;

  second.assign (it,first.end()-1); // the 5 central values of first

  int myints[] = {1776,7,4};
  third.assign (myints,myints+3);   // assigning from array.

  std::cout << "Size of first: " << int (first.size()) << '\n';
  std::cout << "Size of second: " << int (second.size()) << '\n';
  std::cout << "Size of third: " << int (third.size()) << '\n';
  return 0;
}

Output:
Size of first: 7
Size of second: 5
Size of third: 3

Complexity

Linear on initial and final sizes (destructions, constructions).
Additionally, in the range version (1), if InputIterator is not at least of a forward iterator category (i.e., it is just an input iterator) the new capacity cannot be determined beforehand and the operation incurs in additional logarithmic complexity in the new size (reallocations while growing).

Iterator validity

All iterators, pointers and references related to this container are invalidated.

Data races

All copied elements are accessed.
The container is modified.
All contained elements are modified.

Exception safety

Basic guarantee: if an exception is thrown, the container is in a valid state.
If allocator_traits::construct is not supported with the appropriate arguments for the element constructions, or if the range specified by [first,last) is not valid, it causes undefined behavior.

See also

vector::operator=
Assign content (public member function )
vector::resize
Change size (public member function )

C++