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### 3004. §[string.capacity] and §[vector.capacity] should specify time complexity for `capacity()`

Section: 23.4.3.5 [string.capacity], 24.3.11.3 [vector.capacity] Status: C++20 Submitter: Andy Giese Opened: 2017-07-24 Last modified: 2021-02-25

Priority: 0

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Discussion:

`basic_string` and `vector` both have a `capacity` function that returns the size of the internally allocated buffer. This function does not specify a time complexity nor does it have an implied time complexity. However, given the complexities for `data()` to be `𝒪(1)` and `size()` to be `𝒪(1)`, we can imagine that it's reasonable to also require `capacity()` to be `𝒪(1)`, since the implementation will most likely be caching the size of the allocated buffer so as to check for the need to reallocate on insertion.

[ 2017-11-01 Moved to Tentatively Ready after 10 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4659.

1. Edit 23.4.3.5 [string.capacity] as indicated:

   >     size_type capacity() const noexcept;

   > > -9- Returns: The size of the allocated storage in the string.

   > > -?- Complexity: Constant time.

2. Edit 24.3.11.3 [vector.capacity] as indicated:

   >     size_type capacity() const noexcept;

   > > -1- Returns: The total number of elements that the vector can hold without requiring reallocation.

   > > -?- Complexity: Constant time.