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.
| new_cap | - | new capacity of the container |
| Type requirements | ||
-T must meet the requirements of MoveInsertable. |
||
(none).
std::length_error if new_cap > max_size(). Allocator::allocate() (typically std::bad_alloc) At most linear in the size() of the container.
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.
#include <cstddef>
#include <new>
#include <vector>
#include <iostream>
// minimal C++11 allocator with debug output
template <class Tp>
struct NAlloc {
typedef Tp value_type;
NAlloc() = default;
template <class T> NAlloc(const NAlloc<T>&) {}
Tp* allocate(std::size_t n) {
n *= sizeof(Tp);
std::cout << "allocating " << n << " bytes\n";
return static_cast<Tp*>(::operator new(n));
}
void deallocate(Tp* p, std::size_t n) {
std::cout << "deallocating " << n*sizeof*p << " bytes\n";
::operator delete(p);
}
};
template <class T, class U>
bool operator==(const NAlloc<T>&, const NAlloc<U>&) { return true; }
template <class T, class U>
bool operator!=(const NAlloc<T>&, const NAlloc<U>&) { return false; }
int main()
{
int sz = 100;
std::cout << "using reserve: \n";
{
std::vector<int, NAlloc<int>> v1;
v1.reserve(sz);
for(int n = 0; n < sz; ++n)
v1.push_back(n);
}
std::cout << "not using reserve: \n";
{
std::vector<int, NAlloc<int>> v1;
for(int n = 0; n < sz; ++n)
v1.push_back(n);
}
}Possible output:
using reserve: allocating 400 bytes deallocating 400 bytes not using reserve: allocating 4 bytes allocating 8 bytes deallocating 4 bytes allocating 16 bytes deallocating 8 bytes allocating 32 bytes deallocating 16 bytes allocating 64 bytes deallocating 32 bytes allocating 128 bytes deallocating 64 bytes allocating 256 bytes deallocating 128 bytes allocating 512 bytes deallocating 256 bytes deallocating 512 bytes
| returns the number of elements that can be held in currently allocated storage (public member function) |
|
| returns the maximum possible number of elements (public member function) |
|
| changes the number of elements stored (public member function) |
|
|
(C++11) | reduces memory usage by freeing unused memory (public member function) |
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