Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | string‎ | basic string
 
 
 
std::basic_string
Member functions
Element access
Iterators
Capacity
basic_string::reserve
Operations
Search
Constants
Deduction guides (C++17)
Non-member functions
I/O
Comparison
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Numeric conversion
(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Helper classes
 
(1)
void reserve( size_type new_cap = 0 );
(until C++20)
constexpr void reserve( size_type new_cap );
(since C++20)
void reserve();
(2) (since C++20)
(deprecated)
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)

Contents

[edit] Parameters

new_cap - new capacity of the string

[edit] Return value

(none)

[edit] Exceptions

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.

[edit] Complexity

At most linear in the size() of the string

[edit] Example

#include <cassert>
#include <iostream>
#include <string>
 
int main()
{
    std::string s;
    const std::string::size_type new_capacity{ 100u };
    std::cout << "Before: " << s.capacity() << "\n";
 
    s.reserve(new_capacity);
    std::cout << "After:  " << s.capacity() << "\n";
    assert(new_capacity <= s.capacity());
 
    // observing the capacity growth factor
    auto cap{ s.capacity() };
    for (int check{}; check != 4; ++check) {
        while(cap == s.capacity())
            s += '$';
        cap = s.capacity();
        std::cout << "New capacity: " << cap << '\n';
    }
 
    // s.reserve(); //< deprecated in C++20, use:
    s.shrink_to_fit();
    std::cout << "After:  " << s.capacity() << "\n";
}

Possible output:

Before: 15
After:  100
New capacity: 200
New capacity: 400
New capacity: 800
New capacity: 1600
After:  801

[edit] See also

returns the number of characters that can be held in currently allocated storage
(public member function) [edit]
changes the number of characters stored
(public member function) [edit]