std::vector<T,Allocator>::reserve

#include <cstddef>
#include <new>
#include <vector>
#include <iostream>
 
// 带调试输出的最小 C++11 分配器
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(Tp) << " 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);
    }
}