21 Strings library [strings]

21.3 String classes [string.classes]

21.3.3 Class template basic_­string [basic.string]

21.3.3.5 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.
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.
constexpr void resize(size_type n, charT c);
Effects: Alters the value of *this as follows:
  • If n <= size(), erases the last size() - n elements.
  • If n > size(), appends n - size() copies of c.
constexpr void resize(size_type n);
Effects: Equivalent to resize(n, charT()).
template<class Operation> constexpr void resize_and_overwrite(size_type n, Operation op);
Let
  • o = size() before the call to resize_­and_­overwrite.
  • k be min(o, n).
  • p be a charT*, 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]).
  • OP be the expression std​::​move(op)(p, n).
  • r = OP.
Mandates: OP has an integer-like type ([iterator.concept.winc]).
Preconditions:
  • OP does not throw an exception or modify p or n.
  • .
  • .
  • 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.
constexpr size_type capacity() const noexcept;
Returns: The size of the allocated storage in the string.
Complexity: Constant time.
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.
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]
constexpr void clear() noexcept;
Effects: Equivalent to: erase(begin(), end());
[[nodiscard]] constexpr bool empty() const noexcept;
Effects: Equivalent to: return size() == 0;