不重新分配内存的std::set的替代方案

使用自定义分配器似乎是减少构建和发布std::set<...>所花费的时间的一种方法。下面是一个简单分配器的完整演示，以及对结果时间的程序分析。

#include <algorithm>
#include <chrono>
#include <cstdlib>
#include <iostream>
#include <iterator>
#include <memory>
#include <set>
#include <vector>
// ----------------------------------------------------------------------------
template <typename T, std::size_t pool_size = 1024>
class pool_allocator
{
private:
    std::vector<T*> d_pools;
    T*              d_next;
    T*              d_end;
public:
    template <typename O>
    struct rebind {
        typedef pool_allocator<O, pool_size> other;
    };
    pool_allocator(): d_next(), d_end() {}
    ~pool_allocator() {
        std::for_each(this->d_pools.rbegin(), this->d_pools.rend(),
                      [](T* memory){ operator delete(memory); });
    }
    typedef T value_type;
    T*   allocate(std::size_t n) {
        if (std::size_t(this->d_end - this->d_next) < n) {
            if (pool_size < n) {
                // custom allocation for bigger number of objects
                this->d_pools.push_back(static_cast<T*>(operator new(sizeof(T) * n)));
                return this->d_pools.back();
            }
            this->d_pools.push_back(static_cast<T*>(operator new(sizeof(T) * pool_size)));
            this->d_next = this->d_pools.back();
            this->d_end  = this->d_next + pool_size;
        }
        T* rc(this->d_next);
        this->d_next += n;
        return rc;
    }
    void deallocate(T*, std::size_t) {
        // this could try to recycle buffers
    }
};
// ----------------------------------------------------------------------------
template <typename Allocator>
void time(char const* name, std::vector<int> const& random) {
    std::cout << "running " << name << std::flush;
    using namespace std::chrono;
    high_resolution_clock::time_point start(high_resolution_clock::now());
    std::size_t size(0);
    {
        std::set<int, std::less<int>, Allocator> values;
        for (int value: random) {
            values.insert(value);
        }
        size = values.size();
    }
    high_resolution_clock::time_point end(high_resolution_clock::now());
    std::cout << ": size=" << size << " time="
              << duration_cast<milliseconds>(end - start).count() << "msn";
}
// ----------------------------------------------------------------------------
int main()
{
    std::cout << "preparing..." << std::flush;
    std::size_t count(10000000);
    std::vector<int> random;
    random.reserve(count);
    std::generate_n(std::back_inserter(random), count, [](){ return std::rand(); });
    std::cout << "donen";
    time<std::allocator<int>>("default allocator      ", random);
    time<pool_allocator<int, 32>>("custom allocator (32)  ", random);
    time<pool_allocator<int, 256>>("custom allocator (256) ", random);
    time<pool_allocator<int, 1024>>("custom allocator (1024)", random);
    time<pool_allocator<int, 2048>>("custom allocator (2048)", random);
    time<pool_allocator<int, 4096>>("custom allocator (4096)", random);
    time<std::allocator<int>>("default allocator      ", random);
}
// results from clang/libc++:
// preparing...done
// running default allocator      : size=10000000 time=13927ms
// running custom allocator (32)  : size=10000000 time=9260ms
// running custom allocator (256) : size=10000000 time=9511ms
// running custom allocator (1024): size=10000000 time=9172ms
// running custom allocator (2048): size=10000000 time=9153ms
// running custom allocator (4096): size=10000000 time=9599ms
// running default allocator      : size=10000000 time=13730ms
// results from gcc/libstdc++:
// preparing...done
// running default allocator      : size=10000000 time=15814ms
// running custom allocator (32)  : size=10000000 time=10868ms
// running custom allocator (256) : size=10000000 time=10229ms
// running custom allocator (1024): size=10000000 time=10556ms
// running custom allocator (2048): size=10000000 time=10392ms
// running custom allocator (4096): size=10000000 time=10664ms
// running default allocator      : size=10000000 time=17941ms

在std::set中使用自定义分配器可能会有所帮助。如果在构造集合之前知道元素的数量，您可以分配一个大小合适的原始内存缓冲区，然后在您的自定义分配器类(使用std::allocator作为基类)中重写allocate方法，以便它返回指向缓冲区中地址的指针，而不是调用new操作符。它仍然需要内存分配，但只需要一次。它可能看起来像这样:

template<class T, size_t S>
class MyAlloc: public allocator<T>
{
    T *buf;
    size_t ptr;
public:
    MyAlloc()
    {
        buf = (T*)malloc(sizeof(T) * S);
        ptr = 0;
    }
    ~MyAlloc()
    {
        free(buf);
    }
    T* allocate(size_t n, allocator<void>::const_pointer hint=0)
    {
        ptr += n;
        return &buf[ptr - n];
    }
    void deallocate(T* p, size_t n)
    {
        //Do nothing.
    }
    template<class T1>
    struct rebind
    {
        typedef MyAlloc<T1, S> other;
    };
};