std::vector::resize(3)

NAME
std::vector::resize - std::vector::resize

Synopsis
   void resize( size_type count ); (1) (constexpr since C++20)
   void resize( size_type count, const value_type& value ); (2) (constexpr since C++20)
   Resizes the container to contain count elements, does nothing if count == size().
   If the current size is greater than count, the container is reduced to its first
   count elements.
   If the current size is less than count,
   1) additional default-inserted elements are appended.
   2) additional copies of value are appended.

Parameters
   count - new size of the container
   value - the value to initialize the new elements with

Type requirements
   -
   T must meet the requirements of MoveInsertable and DefaultInsertable in order to use
   overload (1).
   -
   T must meet the requirements of CopyInsertable in order to use overload (2).

Return value
   (none)

Complexity
   Linear in the difference between the current size and count. Additional complexity
   possible due to reallocation if capacity is less than count.

Exceptions
   If an exception is thrown for any reason, these functions have no effect (strong
   exception safety guarantee). Although not explicitly specified, std::length_error is
   thrown if the capacity required by the new vector would exceed max_size().
   In overload (1), if T's move constructor is not noexcept and T is not
   CopyInsertable into *this, vector will use the throwing move (since C++11)
   constructor. If it throws, the guarantee is waived and the effects are
   unspecified.

Notes
   If value-initialization in overload (1) is undesirable, for example, if the elements
   are of non-class type and zeroing out is not needed, it can be avoided by providing
   a custom Allocator::construct.
   Vector capacity is never reduced when resizing to smaller size because that would
   invalidate all iterators, rather than only the ones that would be invalidated by the
   equivalent sequence of pop_back() calls.

Example
// Run this code
 #include <vector>
 #include <iostream>
 void print(auto rem, const std::vector<int>& c)
 {
     for (std::cout << rem; const int el : c)
         std::cout << el << ' ';
     std::cout << '\n';
 }
 int main()
 {
     std::vector<int> c = {1, 2, 3};
     print("The vector holds: ", c);
     c.resize(5);
     print("After resize up to 5: ", c);
     c.resize(2);
     print("After resize down to 2: ", c);
     c.resize(6, 4);
     print("After resize up to 6 (initializer = 4): ", c);
 }

Output:
 The vector holds: 1 2 3
 After resize up to 5: 1 2 3 0 0
 After resize down to 2: 1 2
 After resize up to 6 (initializer = 4): 1 2 4 4 4 4
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 679 C++98 resize() passed value by value passes by const
                        reference
   LWG 1525 C++98 the behavior of resize(size()) was not specified
                       specified
   LWG 2033 C++11 1. elements were removed by using erase() 1. uses pop_back()
                       2. the type requirements of T were incorrect 2. corrected
   LWG 2066 C++11 overload (1) did not have the exception added
                       safety guarantee of overload (2)

See also
   size returns the number of elements
          (public member function)
   insert inserts elements
          (public member function)
   erase erases elements
          (public member function)