std::vector::reserve

 

C++

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Technical Specifications

 

Containers library

array (C++11)

vector

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list

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std::vector

Member functions

vector::vector

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Capacity

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vector::reserve

vector::capacity

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void reserve( size_type new_cap );

Increase the capacity of the container to a value that's greater or equal to new_cap. If new_cap is greater than the current capacity(), new storage is allocated, otherwise the method does nothing.

If new_cap is greater than capacity(), all iterators, including the past-the-end iterator, and all references to the elements are invalidated. Otherwise, no iterators or references are invalidated.

Parameters

new_cap	-	new capacity of the container
Type requirements
- T must meet the requirements of MoveInsertable.

Return value

(none)

Exceptions

• std::length_error if new_cap > max_size().
• any exception thrown by Allocator::allocate() (typically std::bad_alloc)

Complexity

At most linear in the size() of the container.

Notes

reserve() cannot be used to reduce the capacity of the container, to that end shrink_to_fit() is provided.

Correctly using reserve() can prevent unnecessary reallocations, but inappropriate uses of reserve() (for instance, calling it before every push_back() call) may actually increase the number of reallocations (by causing the capacity to grow linearly rather than exponentially) and result in increased computational complexity and decreased performance.