stdvectorreserve

stdvectorreserve is a member function of the std::vector class in C++. It is used to request that the container capacity be at least enough to contain n elements. If n is greater than the current capacity, reserve causes the container's capacity to grow. If n is less than or equal to the current capacity, reserve does nothing. The capacity of a vector is the number of elements it can hold before needing to reallocate its internal storage.

Calling reserve() is a performance optimization. When a vector grows, it may need to reallocate memory and copy existing elements to a new, larger block of memory. This reallocation can be an expensive operation, especially if it happens many times. By calling reserve() beforehand with an estimate of the number of elements you expect to store, you can reduce or eliminate the number of reallocations.

If the new capacity is greater than the current capacity, all elements in the vector remain valid, and their access numbers are unchanged. If the new capacity is greater than the current capacity, and if the vector has elements, the existing elements are copied to the new storage. The constructor of std::vector guarantees that reserve() does not throw exceptions.