std :: vector :: reserve失效

Question

If I call std::vector::reserve to reserve a certain amount of memory for my vector, will this memory remain allocated until I destroy my vector or are there any method calls (perhaps clear ) that will free my reserved memory?

Edit : I will be reusing the container a large number of times so for performance reason I want to avoid memory allocations. It is for this reason I reserve memory up front so I want to be certain I do nothing to lose the allocated memory.

Answer 1

clear only affects the size, not the capacity. shrink_to_fit in C++11 may be what you are looking for.

Answer 2

Edit: I will be reusing the container a large number of times so for performance reason I want to avoid memory allocations. It is for this reason I reserve memory up front so I want to be certain I do nothing to lose the allocated memory.

You need only to avoid two things:

1. shrink_to_fit , but it is only a request to free memory, it is not a must for vector to actually do this.

2. Only swap with empty vector will change capacity for sure (see See ideone ):

vector<int> v;
v.reserve(100);
vector<int>().swap(v);
ASSERT(v.capacity() == 0);

Neither pop_back , clear nor resize to smaller size will reduce vector capacity.

BTW, consider to use std::array<> .