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std::basic_string<CharT,Traits,Allocator>::reserve - cppreference.com

void reserve( size_type new_cap = 0 );

constexpr void reserve( size_type new_cap );

void reserve();

Informs a

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).

• If new_cap is less than or equal to the current capacity(), there is no effect.

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

A non-binding shrink-to-fit request. After this call,

has an unspecified value greater than or equal to

.

(none)

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.

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

At most linear in the size() of the string.

#include <cassert>
#include <iostream>
#include <string>
 
int main()
{
    std::string s;
    std::cout << "1) Initially: " << s.capacity() << '\n';
 
    const std::string::size_type new_cap{101u};
    s.reserve(new_cap);
    assert(s.capacity() >= new_cap);
    std::cout << "2) After reserve(" << new_cap << "): " << s.capacity() << '\n';
 
    // 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 << (3) + check << ") Capacity: " << cap << '\n';
    }
 
//  s.reserve(); // deprecated/removed in C++20/26, use:
    s.shrink_to_fit();
    std::cout << "7) After shrink_to_fit: " << s.capacity() << '\n';
}

Possible output:

1) Initially: 15
2) After reserve(101): 101
3) Capacity: 202
4) Capacity: 404
5) Capacity: 808
6) Capacity: 1616
7) After shrink_to_fit: 809

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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