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

void shrink_to_fit();		(until C++20)
constexpr void shrink_to_fit();		(since C++20)

Requests the removal of unused capacity.

It is a non-binding request to reduce capacity() to size(). It depends on the implementation if the request is fulfilled.

If (and only if) reallocation takes place, all pointers, references, and iterators are invalidated.

Parameters

(none).

Return value

(none).

Complexity

(unspecified).	(until C++17)
Linear in the size of the string.	(since C++17)

Notes

In libstdc++, shrink_to_fit() is not available in C++98 mode.

Example

#include <iostream>
#include <string>
 
int main()
{
    std::string s;
    std::cout << "Size of std::string is " << sizeof s << " bytes\n"
        << "Default-constructed capacity is " << s.capacity() 
        << " and size is " << s.size() << '\n';
 
    for (int i = 0; i < 42; i++)
        s.append(" 42 ");
    std::cout << "Capacity after 42 appends is " << s.capacity() 
        << " and size is " << s.size() << '\n';
 
    s.clear();
    std::cout << "Capacity after clear() is " << s.capacity() 
        << " and size is " << s.size() << '\n';
 
    s.shrink_to_fit();
    std::cout << "Capacity after shrink_to_fit() is " << s.capacity() 
        << " and size is " << s.size() << '\n';
}

Possible output:

GCC output:
Size of std::string is 32 bytes
Default-constructed capacity is 15 and size 0
Capacity after 42 appends is 240 and size 168
Capacity after clear() is 240 and size 0
Capacity after shrink_to_fit() is 15 and size 0
 
clang output (with -stdlib=libc++):
Size of std::string is 24 bytes
Default-constructed capacity is 22 and size is 0
Capacity after 42 appends is 191 and size is 168
Capacity after clear() is 191 and size is 0
Capacity after shrink_to_fit() is 22 and size is 0

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 755	C++98	`std::string` lacked explicit shrink-to-fit operations	provided

sizelength	returns the number of characters (public member function)
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)