Difference between revisions of "cpp/container/vector/reserve"

Revision as of 16:19, 2 November 2012

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 end() iterator) and all references to the elements are invalidated. Otherwise, no iterators or references are invalidated.

Parameters

Return value

(none)

Exceptions

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

If an exception is thrown, this function has no effect (strong exception guarantee).

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.

Example

#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);
    }
}

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

See also