18.4.2 内存池代码的改进

#include 
#include 
#include 
using namespace std;//专门的内存池类或内存分配器
//使用本类的类sizeof()必须大于等于4
class myallocator
{
public://释放内存池~myallocator(){while (m_FreePosi){obj *tmp = m_FreePosi;m_FreePosi = m_FreePosi->next;free((void *)tmp);}}//分配内存接口void *allocate(size_t size){obj *tmplink;if (m_FreePosi == nullptr){size_t realsize = m_sTrunkCout * size;m_FreePosi = (obj *)malloc(realsize);tmplink = m_FreePosi;for (int i = 0; i < m_sTrunkCout - 1; ++i){tmplink->next = (obj *)((char *)tmplink + size);tmplink = tmplink->next;}tmplink->next = nullptr;m_sTrunkCout *= 2;}tmplink = m_FreePosi;m_FreePosi = m_FreePosi->next;return tmplink;}//释放内存接口void deallocate(void *p){((obj *)p)->next = m_FreePosi;m_FreePosi = (obj *)p;}private://类内结构，只能类内使用struct obj{struct obj *next; // next就是嵌入式指针};int m_sTrunkCout = 5;obj *m_FreePosi = nullptr;
};class A
{
public://必须保证sizeof(A)凑够4字节int m_i;int m_j;public:static myallocator myalloc;static void *operator new(size_t size){return myalloc.allocate(size);}static void operator delete(void *p){myalloc.deallocate(p);}
};
myallocator A::myalloc;int main()
{{cout << sizeof(myallocator) << endl;cout << sizeof(A) << endl;A *mypa[100];for (int i = 0; i < 15; ++i){mypa[i] = new A();printf("%p\n", mypa[i]);}for (int i = 0; i < 15; ++i){delete mypa[i];}}cout << "Over!\n";return 0;
}

#include 
#include 
#include 
using namespace std;//专门的内存池类或内存分配器
//使用本类的类sizeof()必须大于等于4
class myallocator
{
public://释放内存池~myallocator(){while (m_FreePosi){obj *tmp = m_FreePosi;m_FreePosi = m_FreePosi->next;free((void *)tmp);}}//分配内存接口void *allocate(size_t size){obj *tmplink;if (m_FreePosi == nullptr){size_t realsize = m_sTrunkCout * size;m_FreePosi = (obj *)malloc(realsize);tmplink = m_FreePosi;for (int i = 0; i < m_sTrunkCout - 1; ++i){tmplink->next = (obj *)((char *)tmplink + size);tmplink = tmplink->next;}tmplink->next = nullptr;m_sTrunkCout *= 2;}tmplink = m_FreePosi;m_FreePosi = m_FreePosi->next;return tmplink;}//释放内存接口void deallocate(void *p){((obj *)p)->next = m_FreePosi;m_FreePosi = (obj *)p;}private://类内结构，只能类内使用struct obj{struct obj *next; // next就是嵌入式指针};int m_sTrunkCout = 5;obj *m_FreePosi = nullptr;
};#define DECLARE_POOL_ALLOC()               \
public:                                    \static void *operator new(size_t size) \{                                      \return myalloc.allocate(size);     \}                                      \static void operator delete(void *p)   \{                                      \myalloc.deallocate(p);             \}                                      \static myallocator myalloc;#define IMPLEMENT_POOL_ALLOC(classname) \myallocator classname::myalloc;class A
{DECLARE_POOL_ALLOC();public://必须保证sizeof(A)凑够4字节int m_i;int m_j;
};
IMPLEMENT_POOL_ALLOC(A);int main()
{{cout << sizeof(myallocator) << endl;cout << sizeof(A) << endl;A *mypa[100];for (int i = 0; i < 15; ++i){mypa[i] = new A();printf("%p\n", mypa[i]);}for (int i = 0; i < 15; ++i){delete mypa[i];}}cout << "Over!\n";return 0;
}

18.5 重载全局new/delete、定位new及重载

18.5.1 重载全局operator new/delete ([]) 操作符

//重载全局operator new
void * operator new(size_t size){return malloc(size);
}//重载全局operator new[]
void * operator new[](size_t size){return malloc(size);
}//重载全局operator delete
void * operator delete(void * p){return free(p);
}//重载全局operator delete[]
void * operator delete[](void * p){return free(p);
}int *pi = new int(12);
delete pi;
char *pc = new char[10](0);
delete[] pc;A *p = new A();
delete p;
A *pa = new A[3]();
delete[] pa;

一般重载类中operator new/delete ([])。

18.5.2 定位new（placement new）

placement new无对应的placement delete，在已经分配的原始内存中初始化一个对象。

• 定位new不分配内存，使用定位new前内存必须先分配好；
• 初始化对象，就是调用对象的构造函数。

new(地址) 类类型(参数);

class PLA{
public:int m_a;PLA():m_a(0){cout<<"PLA()"<cout<<"PLA(int)"<~PLA();//根据需要调用析构函数
delete[](void *)po1;//释放内存
//delete[] p1;//同上//先分配内存
void *p2 = (void *)new char[sizeof(PLA)](0);
//定位new，调用有参构造函数，不额外分配内存
PLA *po2 = new(p2) PLA(12);
po2->~PLA();//根据需要调用析构函数
delete[](void *)po2;//释放内存
//delete[] p2;//同上

定位new调用关系

PLA *pa = new(已经分配好的内存首地址) PLA();//定位new操作符operator new();//函数，内部没有调用mallocPLA::PLA();//调用构造函数

定位new调用的operator new操作符的重载代码。

public:void * operator new(size_t, void *p){//多一个p，指向已经分配好的内存首地址return p;}

18.5.3 多种版本的operator new 重载

第一个参数固定size_t（sizeof(对象)），其他参数指定。

//无内存分配，无类构造函数调用
PLA *po2 = new(1234, 56) PLA(12);public://第一个参数，系统默认传递sizeof(PLA)void * operator new(size_t size, int t1, int t2){return NULL;}

会出现警告：void *PLA::operator new(size_t, int, int)未找到匹配的删除运算符。

可以增加对应的operator delete重载以避免警告（非必需）。

public:void  operator delete(void* p, int t1, int t2){return;}