【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