std::basic_string<CharT,Traits,Allocator>::reserve (3) - Linux Manuals

std::basic_string<CharT,Traits,Allocator>::reserve: std::basic_string<CharT,Traits,Allocator>::reserve


std::basic_string<CharT,Traits,Allocator>::reserve - std::basic_string<CharT,Traits,Allocator>::reserve


void reserve( size_type new_cap = 0 ); (1) (until C++20)
void reserve( size_type new_cap); (1) (since C++20)
void reserve(); (2) (since C++20)

1) Informs a std::basic_string object of a planned change in size, so that it can manage the storage allocation appropriately.

      * If new_cap is greater than the current capacity(), new storage is allocated, and capacity() is made equal or greater than new_cap.

      * If new_cap is less than the current capacity(), this is a non-binding shrink request.
      * If new_cap is less than the current size(), this is a non-binding shrink-to-fit request (until C++20)
        equivalent to shrink_to_fit()
        (since C++11).

      * If new_cap is less than or equal to the current capacity(), there is no effect. (since C++20)

If a capacity change takes place, all iterators and references, including the past-the-end iterator, are invalidated.

2) A call to reserve with no argument is a non-binding shrink-to-fit request. After this call, capacity() has an unspecified value greater than or equal to size(). (since C++20)


new_cap - new capacity of the string

Return value



Throws std::length_error if new_cap is greater than max_size()
May throw any exceptions thrown by std::allocator_traits<Allocator>::allocate(), such as std::bad_alloc.


At most linear in the size() of the string


// Run this code

  #include <cassert>
  #include <string>

  int main()
      std::string s;
      std::string::size_type new_capacity{ 100u };
      assert(new_capacity > s.capacity());

      assert(new_capacity <= s.capacity());

See also

         returns the number of characters that can be held in currently allocated storage
capacity (public member function)