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

 

C++

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std::basic_string

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	(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)

Parameters

new_cap

-

new capacity of the string

Return value

(none)

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.

Complexity

At most linear in the size() of the string

Example

Run this code

#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

See also

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