# std::vector<T,Allocator>::shrink_to_fit

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< cpp‎ | container‎ | vector

void shrink_to_fit(); |  | (constexpr 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 whether the request is fulfilled.

If reallocation occurs, all iterators (including the `end()` iterator) and all references to the elements are invalidated. If no reallocation occurs, no iterators or references are invalidated.

If `T` is not MoveInsertable into std::vector <T, Allocator>, the behavior is undefined. | (since C++11)

### Complexity

At most linear in the size of the container.

### Exceptions

If an exception is thrown other than by the move constructor of a non-CopyInsertable `T`, there are no effects.

(since C++11)

### Notes

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

### Example

Run this code

    #include <iostream>
    #include <vector>

    int main()
    {
        std::vector<int> v;
        std::cout << "Default-constructed capacity is " << v.capacity() << '\n';
        v.resize(100);
        std::cout << "Capacity of a 100-element vector is " << v.capacity() << '\n';
        v.resize(50);
        std::cout << "Capacity after resize(50) is " << v.capacity() << '\n';
        v.shrink_to_fit();
        std::cout << "Capacity after shrink_to_fit() is " << v.capacity() << '\n';
        v.clear();
        std::cout << "Capacity after clear() is " << v.capacity() << '\n';
        v.shrink_to_fit();
        std::cout << "Capacity after shrink_to_fit() is " << v.capacity() << '\n';
        for (int i = 1000; i < 1300; ++i)
            v.push_back(i);
        std::cout << "Capacity after adding 300 elements is " << v.capacity() << '\n';
        v.shrink_to_fit();
        std::cout << "Capacity after shrink_to_fit() is " << v.capacity() << '\n';
    }

Possible output:

    Default-constructed capacity is 0
    Capacity of a 100-element vector is 100
    Capacity after resize(50) is 100
    Capacity after shrink_to_fit() is 50
    Capacity after clear() is 50
    Capacity after shrink_to_fit() is 0
    Capacity after adding 300 elements is 512
    Capacity after shrink_to_fit() is 300

### 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::vector` lacked explicit shrink-to-fit operations | provided
LWG 2033 | C++98 C++11 | 1. the complexity requirement was missing (C++98) 2. `T` was not required to be MoveInsertable (C++11) | 1. added 2. required
LWG 2223 | C++98 C++11 | 1. references, pointers, and iterators were not invalidated (C++98) 2. there was no exception safety guarantee (C++11) | 1. they may be invalidated 2. added

### See also

size | returns the number of elements (public member function)
capacity | returns the number of elements that can be held in currently allocated storage (public member function)

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