< cpp‎ | string‎ | basic string
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 equivalent to shrink_to_fit() (since C++11).
(until C++20)
  • 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


#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
(public member function)