0.手写实现简单内存池.md

#include <chrono>
#include <iostream>

using namespace std;

#define START_LOG_TIME()                                                              \
    std::chrono::steady_clock::time_point t1 =                                        \
            std::chrono::steady_clock::now();                                         \
    std::chrono::steady_clock::time_point t2 = t1;                                    \
    std::chrono::duration<double> time_used =                                         \
            std::chrono::duration_cast<std::chrono::duration<double>>(t2 - t1);
#define PRINT_COST_TIME(name)                                                         \
    t2 = std::chrono::steady_clock::now();                                            \
    time_used = std::chrono::duration_cast<std::chrono::duration<double>>(t2 - t1);   \
    std::cout << name << " TIME COST: " << (time_used.count() * 1000)                 \
              << " ms." << std::endl;                                                 \
    t1 = t2;

// #define MYMEMPOOL

class A {
public:
	static void* operator new(size_t size);
	static void operator delete(void* phead);
	static int new_cnt_; // 每分配一次类A+1
	static int mempool_cnt_; // 每分配一次内存池+1

private:
	A* next;
	static A* free_pos_; // 总是指向一块可以分配出去的内存的首地址
	static int trunk_cnt_; // 一次分配类A的内存的倍数，作为内存池的大小
};

void* A::operator new(size_t size) {
#ifdef MYMEMPOOL
	A* ptr = (A*)malloc(size);
	return ptr;
#endif
	A* tmp_link = nullptr;
	if (free_pos_ == nullptr) {
		size_t real_size = trunk_cnt_ * size;
		free_pos_ = reinterpret_cast<A*>(new char[real_size]); // 这里使用传统new，调用传统底层malloc
		tmp_link = free_pos_;

		for (; tmp_link != &free_pos_[trunk_cnt_ - 1]; ++tmp_link) {
			tmp_link->next = tmp_link + 1;
		}
		tmp_link->next = nullptr;
		++mempool_cnt_;
		cout << "mempool_cnt_:" << mempool_cnt_ << endl;
	}
	tmp_link = free_pos_;
	free_pos_ = free_pos_->next;
	++new_cnt_;
	cout << "new_cnt_:" << new_cnt_ << endl;
	return tmp_link;
}

void A::operator delete(void* phead) {
#ifdef MYMEMPOOL
	free(phead);
#endif
	(static_cast<A*>(phead))->next = free_pos_;
	free_pos_ = static_cast<A*>(phead); // 不是真正的释放，而是还给内存池，以便下次申请使用
}

int A::new_cnt_ = 0;
int A::mempool_cnt_ = 0;

A* A::free_pos_ = nullptr;
int A::trunk_cnt_ = 5; // 一次分配5倍类A的内存，作为内存池的大小

int main() {
	START_LOG_TIME();
	for (int i = 0; i < 10; i++) {
		A* pa = new A();
	}
	PRINT_COST_TIME("mempool test");
}

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