23 Strings library [strings]

23.4 String classes [string.classes]

23.4.3 Class template basic_string [basic.string]

23.4.3.5 Capacity [string.capacity]

constexpr size_type size() const noexcept;
constexpr size_type length() const noexcept;

Returns: A count of the number of char-like objects currently in the string.

Complexity: Constant time.

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constexpr size_type max_size() const noexcept;

Returns: The largest possible number of char-like objects that can be stored in a basic_string.

Complexity: Constant time.

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constexpr void resize(size_type n, charT c);

Effects: Alters the value of *this as follows:

(5.1)
If n <= size(), erases the last size() - n elements.
(5.2)
If n > size(), appends n - size() copies of c.

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constexpr void resize(size_type n);

Effects: Equivalent to resize(n, charT()).

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template<class Operation> constexpr void resize_and_overwrite(size_type n, Operation op);

Let

(7.1)
o = size() before the call to resize_and_overwrite.
(7.2)
k be min(o, n).
(7.3)
p be a value of type charT* or charT* const, such that the range [p, p + n] is valid and this->compare(0, k, p, k) == 0 is true before the call.

The values in the range [p + k, p + n] may be indeterminate ([basic.indet]).
(7.4)
m be a value of type size_type or const size_type equal to n.
(7.5)
OP be the expression std::move(op)(p, m).
(7.6)
r = OP.

Mandates: OP has an integer-like type ([iterator.concept.winc]).

Preconditions:

(9.1)
OP does not throw an exception or modify p or m.
(9.2)
r ≥ 0.
(9.3)
r ≤ m.
(9.4)
After evaluating OP there are no indeterminate values in the range [p, p + r).

Effects: Evaluates OP, replaces the contents of *this with [p, p + r), and invalidates all pointers and references to the range [p, p + n].

Recommended practice: Implementations should avoid unnecessary copies and allocations by, for example, making p a pointer into internal storage and by restoring *(p + r) to charT() after evaluating OP.

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constexpr size_type capacity() const noexcept;

Returns: The size of the allocated storage in the string.

Complexity: Constant time.

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constexpr void reserve(size_type res_arg);

Effects: A directive that informs a basic_string of a planned change in size, so that the storage allocation can be managed accordingly.

After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise.

Reallocation happens at this point if and only if the current capacity is less than the argument of reserve().

Throws: length_error if res_arg > max_size() or any exceptions thrown by allocator_traits <Allocator>::allocate.

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constexpr void shrink_to_fit();

Effects: shrink_to_fit is a non-binding request to reduce capacity() to size().

[Note 1:

The request is non-binding to allow latitude for implementation-specific optimizations.

— end note]

It does not increase capacity(), but may reduce capacity() by causing reallocation.

Complexity: If the size is not equal to the old capacity, linear in the size of the sequence; otherwise constant.

Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.

[Note 2:

If no reallocation happens, they remain valid.

— end note]

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constexpr void clear() noexcept;

Effects: Equivalent to: erase(begin(), end());

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[[nodiscard]] constexpr bool empty() const noexcept;

Effects: Equivalent to: return size() == 0;