24 Containers library [containers]

24.3 Sequence containers [sequences]

24.3.11 Class template vector [vector]

24.3.11.3 Capacity [vector.capacity]

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constexpr size_type capacity() const noexcept;
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Returns: The total number of elements that the vector can hold without requiring reallocation.
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Complexity: Constant time.
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constexpr void reserve(size_type n);
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Preconditions: T is Cpp17MoveInsertable into *this.
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Effects: A directive that informs a vector of a planned change in size, so that it can manage the storage allocation 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().
If an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable type, there are no effects.
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Throws: length_error if n > max_size().210
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Complexity: It does not change the size of the sequence and takes at most linear time in the size of the sequence.
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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 1: 
If no reallocation happens, they remain valid.
— end note]
No reallocation shall take place during insertions that happen after a call to reserve() until an insertion would make the size of the vector greater than the value of capacity().
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constexpr void shrink_to_fit();
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Preconditions: T is Cpp17MoveInsertable into *this.
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Effects: shrink_to_fit is a non-binding request to reduce capacity() to size().
[Note 2: 
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.
If an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T there are no effects.
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Complexity: If reallocation happens, linear in the size of the sequence.
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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 3: 
If no reallocation happens, they remain valid.
— end note]
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constexpr void swap(vector& x) noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value || allocator_traits<Allocator>::is_always_equal::value);
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Effects: Exchanges the contents and capacity() of *this with that of x.
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Complexity: Constant time.
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constexpr void resize(size_type sz);
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Preconditions: T is Cpp17MoveInsertable and Cpp17DefaultInsertable into *this.
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Effects: If sz < size(), erases the last size() - sz elements from the sequence.
Otherwise, appends sz - size() default-inserted elements to the sequence.
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Remarks: If an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T there are no effects.
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constexpr void resize(size_type sz, const T& c);
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Preconditions: T is Cpp17CopyInsertable into *this.
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Effects: If sz < size(), erases the last size() - sz elements from the sequence.
Otherwise, appends sz - size() copies of c to the sequence.
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Remarks: If an exception is thrown there are no effects.
210)210)
reserve() uses Allocator​::​allocate() which can throw an appropriate exception.