自定義C++ STL內存分配器

第一種 自定義allocator

C++03時代的STL容器使用的是被稱為分配器allocator的內存管理模塊。
allocator是無狀態（stateless）的，定義里沒有成員變量，全是成員函數和一些typedef。
自定義allocator細節很多，尤其是那個rebind。
allocator是個畫蛇添足的設計，提供了不必要的靈活性，增加了復雜度，增加了心智負擔。

stl_mem.h

#pragma once

#include <iostream>
#include <list>
#include <map>
#include <unordered_map>
#include <vector>
#include <set>
#include <algorithm>
#include <mutex>
using namespace std;
namespace MEM_STL {
    struct memNode
    {
        memNode *nextnode;
    };

    class mempool
    {
    public:
        static void *alloc(int size)
        {
            printf("alloc:%d\n", size);

            lock_guard<mutex> guard(m_mutex);

            int index = getindex(size);
            int realsize = 1 << (index + 1);
            if (m_free_head[index] == NULL)
            {
                return malloc(realsize);
            }
            else
            {
                void *p = m_free_head[index];
                m_free_head[index] = m_free_head[index]->nextnode;
                return p;
            }

            return NULL;
        }
        static void delloc(void *ptr, int size)
        {
            printf("delloc:%d\n", size);

            lock_guard<mutex> guard(m_mutex);

            int index = getindex(size);
            memNode *pNew = (memNode *)ptr;
            pNew->nextnode = m_free_head[index];
            m_free_head[index] = pNew;
        }
        static void report()
        {
            lock_guard<mutex> guard(m_mutex);
            printf("mempool report\n");
            for (int i = 0; i < 32; ++i)
            {
                int n = 0;
                auto p = m_free_head[i];
                while (p)
                {
                    n++;
                    p = p->nextnode;
                }
                printf("index|%02d|len|%d\n", i, n);
            }
        }
    private:
        static int getindex(int size)
        {
            static const unsigned int sizetable[32] =
            {
                1 << 1, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7, 1 << 8, 1 << 9, 1 << 10,
                1 << 11, 1 << 12, 1 << 13, 1 << 14, 1 << 15, 1 << 16, 1 << 17, 1 << 18, 1 << 19, 1 << 20,
                1 << 21, 1 << 22, 1 << 23, 1 << 24, 1 << 25, 1 << 26, 1 << 27, 1 << 28, 1 << 29, 1 << 30,
                (unsigned int)1 << 31, 0xFFFFFFFF
            };

            auto p = lower_bound(sizetable, sizetable + 32, (unsigned int)size);
            return  distance(sizetable, p);
        }

    protected:
        static memNode *m_free_head[32];   //32個指針
        static mutex m_mutex;
    };

    template<class T>
    class my_allocator : public std::allocator<T>
    {
    public:
        typedef std::allocator<T> base_type;
        typedef size_t     size_type;
        typedef T *pointer;

        // 必須要重新定義
        template<class Other>
        struct rebind
        {
            typedef my_allocator<Other> other;
        };

        // 構造函數必須實現
        my_allocator()
        {
        }

        my_allocator(my_allocator<T> const &)
        {
        }

        my_allocator<T> &operator=(my_allocator<T> const &)
        {
            return (*this);
        }

        // 模板
        template<class Other>
        my_allocator(my_allocator<Other> const &)
        {
        }

        // 模板
        template<class Other>
        my_allocator<T> &operator=(my_allocator<Other> const &)
        {
            return (*this);
        }

        // 內存的分配與釋放可以實現為自定義的算法，替換函數體即可
        pointer allocate(size_type count)
        {
            return (pointer)mempool::alloc(count * sizeof(T));
        }

        void deallocate(pointer ptr, size_type count)
        {
            mempool::delloc(ptr, count * sizeof(T));
        }
    };

    template<class _Ty, class _Alloc = my_allocator<_Ty> >
    class m_list : public list<_Ty, _Alloc>
    {
    };

    template<class _Kty, class _Ty, class _Pr = less<_Kty>, class _Alloc = my_allocator<pair<const _Kty, _Ty> > >
    class m_map : public map<_Kty, _Ty, _Pr, _Alloc>
    {
    };

    template<class _Ty, class _Alloc = my_allocator<_Ty>>
    class m_vector : public vector<_Ty, _Alloc>
    {
    };

    template<class _Kty, class _Pr = less<_Kty>, class _Alloc = my_allocator<_Kty> >
    class m_set : public set<_Kty, _Pr, _Alloc>
    {
    };

    template<class _Kty, class _Ty, class _HASH = hash<_Kty>, class _KeyEqual = equal_to<_Kty>, class _Alloc = my_allocator<_Kty>>
    class m_unordered_map : public unordered_map<_Kty, _Ty, _HASH, _KeyEqual, _Alloc>
    {
    };
};

stl_mem.cpp

#include "stl_mem.h"
using namespace MEM_STL;

memNode *mempool::m_free_head[32] = { 0 };
mutex mempool::m_mutex = { };

於是C++17引入了pmr（polymorphic memory resource 多態的內存資源）。

第二種 使用C++17提供的pmr

先看一種非常簡單的內存分配實現：

char *buf = 0;
const int max_size = 1024000;
int len = 0;

void *new_(size_t t){
    void *p = 0;
    if (t + len < max_size){
        p = &buf[len];
        len += (t + 4);
    }
    return p;
}
void free_(void *p){}

class buf_helper{
public:
    buf_helper(){
        buf = new char[max_size];
    }
    ~buf_helper(){
        delete buf;
    }
};
buf_helper g_buf_helper;

new一塊內存，中途只使用不回收，最后一次性delete掉。
pmr就是這種思路。C++17 pmr

點擊查看代碼

#include <iostream>
#include <cstdlib>
#include <iostream>
#include <memory_resource>
#include <vector>

char *buf = 0;
const int max_size = 1024000;
int len = 0;

void *new_(size_t t)
{
    void *p = 0;
    if (t + len < max_size)
    {
        p = &buf[len];
        len += (t + 4);
    }
    return p;
}
void free_(void *p)
{
}

class buf_helper
{
public:
    buf_helper()
    {
        buf = new char[max_size];
    }
    ~buf_helper()
    {
        delete buf;
    }
};
buf_helper g_buf_helper;


class debug_resource : public std::pmr::memory_resource
{
public:
    explicit debug_resource(std::pmr::memory_resource *up = std::pmr::get_default_resource())
        : _upstream{ up }
    {
    }

    void *do_allocate(size_t bytes, size_t alignment) override
    {
        std::cout << " do_allocate(): " << bytes << '\n';
        return _upstream->allocate(bytes, alignment);
    }
    void do_deallocate(void *ptr, size_t bytes, size_t alignment) override
    {
        std::cout << " do_deallocate(): " << bytes << '\n';
        _upstream->deallocate(ptr, bytes, alignment);
    }
    bool do_is_equal(const std::pmr::memory_resource &other) const noexcept override
    {
        std::cout << " do_is_equal(): " << '\n';
        return this == &other;
    }

private:
    std::pmr::memory_resource *_upstream;
};

int main()
{
    char buffer[32] = {};
    std::fill_n(std::begin(buffer), std::size(buffer) - 1, '_');
    std::cout << buffer << '\n';
    debug_resource dr;

    std::pmr::monotonic_buffer_resource pool{ std::data(buffer), std::size(buffer) ,&dr };

    std::pmr::vector<char> vec{ &pool };
    //vec.reserve(26);

    for (char ch = 'a'; ch <= 'z'; ++ch)
    {
        vec.push_back(ch);
        std::cout << buffer << '\n';

    }

    std::cout << buffer << '\n';

}

    

核心代碼

char buffer[32] = {};
memory_resource dr;
std::pmr::monotonic_buffer_resource pool{ std::data(buffer), std::size(buffer) ,&dr };
std::pmr::vector<char> vec{ &pool };

pmr::memory_resource 決定內存來源

monotonic buffer resource
unsynchronized_pool_resource
synchronized_pool_resource

pmr::polymorphic_allocator 負責內存分配

名字中的polymorphic是說：這個新的分配器的行為會表現出多態，與以前的分配器不同。
新分配器和stl分配器兼容。保存了一個memory_resource的指針，所以是有狀態的（statefull）。
外界需要保證memory_resource指針的生命周期。
polymorphic_allocator 就是把Memory_resource適配成Alloctor。

pmr是專門用來提高c++內存性能的！一般應用，對pmr的需求沒有那么強烈。

一句話：內存局部性原理

局部性原理--各類優化的基石

可以細讀這篇文章：游戲引擎開發新感覺！(6) c++17內存管理