C++手写内存池的案例详解

#include <iOStream>
using namespace std;
class DemoClass{
public:
    DemoClass() = default;
    DemoClass(int i):data(i){}
    static void* operator new(size_t size);
    static void operator delete(void *);
    virtual ~DemoClass(){}
private:
    DemoClass *next;
    int data;
    static DemoClass *freeMemHeader;
    static const size_t POOL_SIZE;
};
DemoClass * DemoClass::freeMemHeader = nullptr;
const size_t DemoClass::POOL_SIZE = 24;//设定内存池能容纳24个DemoClass对象
void* DemoClass::operator new(size_t size){
    DemoClass* p;
    if(!freeMemHeader){//freeMemHeader为空，内存池中无空间，分配内存
        size_t pool_mem_bytes = size * POOL_SIZE;//内存池的字节大小 = 每个实例的大小（字节数）* 内存池中能容纳的最大实例数
        freeMemHeader = reinterpret_cast<DemoClass*>(new char[pool_mem_bytes]);//new char[]分配pool_mem_bytes个字节，因为每个char占用1个字节
        cout << "Info:向操作系统申请了" << pool_mem_bytes << "字节的内存。" << endl;
        for(int i = 0;i < POOL_SIZE - 1; ++i){//将内存池中POOL_SIZE个小块内存,串起来。
            freeMemHeader[i].next = &freeMemHeader[i + 1];
        }
        freeMemHeader[POOL_SIZE - 1].next = nullptr;
    }
    p = freeMemHeader;//取内存池（链表）的头部，分配给要实例化的对象
    cout << "Info:从内存池中取了" << size << "字节的内存。" << endl;
    freeMemHeader = freeMemHeader -> next;//从内存池中删去取出的那一小块地址，即更新内存池
    p -> next = nullptr;
    return p;
}
void DemoClass::operator delete(void* p){
    DemoClass* tmp = (DemoClass*) p;
    tmp -> next = freeMemHeader;
    freeMemHeader = tmp;
}

int main(int arGC, char* argv[]){
    cout << "sizeof(DemoClass):" << sizeof(DemoClass) << endl;
    size_t N = 32;
    DemoClass* demos[N];
    for(int i = 0; i < N; ++i){
        demos[i] = new DemoClass(i);
        cout << "address of the ith demo:" << demos[i] << endl;
        cout << endl;
    }
    return 0;
}

#include <iostream>
using namespace std;
class DemoClass{
public:
    DemoClass() = default;
    DemoClass(int i, double p){
        data.num = i;
        data.price = p;
    }
    static void* operator new(size_t size);
    static void operator delete(void *);
    virtual ~DemoClass(){}
private:
    struct DemoData{
        int num;
        double price;
    };
private:
    static DemoClass *freeMemHeader;
    static const size_t POOL_SIZE;
    union {
        DemoClass *next;
        DemoData data;
    };
    
};
DemoClass * DemoClass::freeMemHeader = nullptr;
const size_t DemoClass::POOL_SIZE = 24;//设定内存池能容纳24个DemoClass对象
void* DemoClass::operator new(size_t size){
    DemoClass* p;
    if(!freeMemHeader){//freeMemHeader为空，内存池中无空间，分配内存
        size_t pool_mem_bytes = size * POOL_SIZE;//内存池的字节大小 = 每个实例的大小（字节数）* 内存池中能容纳的最大实例数
        freeMemHeader = reinterpret_cast<DemoClass*>(new char[pool_mem_bytes]);//new char[]分配pool_mem_bytes个字节，因为每个char占用1个字节
        cout << "Info:向操作系统申请了" << pool_mem_bytes << "字节的内存。" << endl;
        for(int i = 0;i < POOL_SIZE - 1; ++i){//将内存池中POOL_SIZE个小块内存,串起来。
            freeMemHeader[i].next = &freeMemHeader[i + 1];
        }
        freeMemHeader[POOL_SIZE - 1].next = nullptr;
    }
    p = freeMemHeader;//取内存池（链表）的头部，分配给要实例化的对象
    cout << "Info:从内存池中取了" << size << "字节的内存。" << endl;
    freeMemHeader = freeMemHeader -> next;//从内存池中删去取出的那一小块地址，即更新内存池
    p -> next = nullptr;
    return p;
}
void DemoClass::operator delete(void* p){
    DemoClass* tmp = (DemoClass*) p;
    tmp -> next = freeMemHeader;
    freeMemHeader = tmp;
}

int main(int argc, char* argv[]){
    cout << "sizeof(DemoClass):" << sizeof(DemoClass) << endl;
    size_t N = 32;
    DemoClass* demos[N];
    for(int i = 0; i < N; ++i){
        demos[i] = new DemoClass(i, i * i);
        cout << "address of the " << i << "th demo:" << demos[i] << endl;
        cout << endl;
    }
    return 0;
}