【C++ Primer | 19】控制内存分配

重载new和delete

1. 测试代码：

 #include<iostream>

 #include<new>

 using namespace std;

 class A {

 public:

     A() { cout << "A constructor" << endl; }

     void* operator new(size_t size)

     {

         cout << "this is A's new" << endl;

         return ::operator new(size);

     }

     void operator delete(void* ptr)

     {

         cout << "this is A's delete" << endl;

         return ::operator delete(ptr);

     }

     ~A() {  cout << "A destructor" << endl; }

 };

 int main()

 {

     A *a = new A;

     delete a;

     return ;

 }

运行结果：

虽然我们不能改变new/delete的行为，但是通过重载operator new() 和 operator delete()我们可以实现自己想要的内存管理方式，这在内存池的实现中十分关键。关于operator new有几点要注意：

（1）当无法满足所要求分配的空间时，则如果有new_handler，则调用new_handler，否则如果没要求不抛出异常（以nothrow参数表达），则执行bad_alloc异常，否则返回0

（2）重载时，返回类型必须声明为void*

（3）重载时，第一个参数类型必须为表达要求分配空间的大小（字节），类型为size_t

（4）重载时，可以带除（3）以外的其它参数

2.测试代码：

 #include <iostream>

 #include <new>

 #include <cstdlib>

 using namespace std;

 void* operator new(size_t size)

 {

     cout << "global Override operator new: " << size << endl;

     void * ptr = malloc(size);

     return ptr;

 }

 void* operator new(size_t size, int flag)

 {

     cout << "global Override operator new: " << size << " " << flag << endl;

     return (::operator new(size));

 }

 void operator delete (void* ptr)

 {

     cout << "global Override operator delete" << endl;

     free(ptr);

     ptr = nullptr;

 }

 void operator delete (void* ptr, int flag)

 {

     cout << "Override operator delete: " << flag << endl;

     ::operator delete(ptr);

     ptr = nullptr;

 }

 int main() {

     int * ptr = new int();

     delete ptr;

     cout << endl << "*********************" << endl << endl;

     ptr = new() int();

     delete ptr;

     return ;

 }

输出结果：

定位new表达式

1. 测试代码：

 #include <iostream>

 using namespace std;

 char addr1[];

 int main()

 {

     cout << "******定位new表达式演示***by David***" << endl;

     char addr2[];

     char *addr3 = new char[];

     cout << "addr1 = " << (void*)addr1 << endl;

     cout << "addr2 = " << (void*)addr2 << endl;

     cout << "addr3 = " << (void*)addr3 << endl;

     int *p = nullptr;

     p = new(addr1)int;  ////把内存分配到静态区

     *p = ;

     cout << (void*)p << "  " << *p << endl;

     p = new(addr2)int; ////把内存分配到栈区

     *p = ;

     cout << (void*)p << "  " << *p << endl;

     p = new(addr3)int;  //把内存分配到堆区

     *p = ;

     cout << (void*)p << "  " << *p << endl;

     return ;

 }

运行结果：

2.测试代码

 #include <iostream>

 #include <string>

 #include <new>

 using namespace std;

 const int BUF = ;

 class JustTesting {

 private:

     string words;

     int number;

 public:

     JustTesting(const string &s = "Just Testing", int n = )

     {

         words = s;

         number = n;

         cout << words << " constructed\n";

     }

     ~JustTesting() { cout << words << " destroyed\n"; }

     void Show() const { cout << words << ", " << number << endl; }

 };

 int main(void)

 {

     char *buffer = new char[BUF];  // get a block of memory

     JustTesting *pc1, *pc2;

     pc1 = new (buffer)JustTesting;  // place object in buffer

     pc2 = new JustTesting("heap1", );  // place object on heap

     cout << "Memory block address:\n" << "buffer: "

         << (void *)buffer << "  heap: " << pc2 << endl;

     cout << "Memory contents: \n";

     cout << pc1 << ": ";

     pc1->Show();

     cout << pc2 << ": ";

     pc2->Show();

     JustTesting *pc3, *pc4;

     pc3 = new (buffer) JustTesting("bad Idea", );

     pc4 = new JustTesting("Heap2", );

     cout << "Memory contents: \n";

     cout << pc3 << ": ";

     pc3->Show();

     cout << pc4 << ": ";

     pc4->Show();

     delete pc2;  // free heap1

     delete pc4;  // free heap2

     delete[] buffer;  // free buffer

     cout << "Done\n";

     return ;

 }

运行结果：

参考资料

定制自己的new和delete：operator new 和 operator delete

巴特西

【C++ Primer | 19】控制内存分配

重载new和delete

定位new表达式

参考资料

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