【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 0;
}

 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(10);
    delete ptr;
    cout << endl << "*********************" << endl << endl;
    ptr = new(20) int(10);
    delete ptr;
    return 0;
}

 输出结果：

 定位new表达式

1. 测试代码：

#include <iostream>
using namespace std;
char addr1[100];
int main()
{
    cout << "******定位new表达式演示***by David***" << endl;
    char addr2[100];
    char *addr3 = new char[100];
    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 = 1;
    cout << (void*)p << "  " << *p << endl;
    
    p = new(addr2)int; ////把内存分配到栈区
    *p = 2;
    cout << (void*)p << "  " << *p << endl;
    
    p = new(addr3)int;  //把内存分配到堆区
    *p = 3;
    cout << (void*)p << "  " << *p << endl;
    return 0;
}

运行结果：

2.测试代码

#include <iostream>
#include <string>
#include <new>

using namespace std;
const int BUF = 512;

class JustTesting {
private:
    string words;
    int number;
public:
    JustTesting(const string &s = "Just Testing", int n = 0)
    {
        words = s; 
        number = n; 
        cout << words << " constructed
";
    }

    ~JustTesting() { cout << words << " destroyed
"; }
    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", 20);  // place object on heap

    cout << "Memory block address:
" << "buffer: "
        << (void *)buffer << "  heap: " << pc2 << endl;
    cout << "Memory contents: 
";
    cout << pc1 << ": ";
    pc1->Show();
    cout << pc2 << ": ";
    pc2->Show();

    JustTesting *pc3, *pc4;
    pc3 = new (buffer) JustTesting("bad Idea", 6);
    pc4 = new JustTesting("Heap2", 10);

    cout << "Memory contents: 
";
    cout << pc3 << ": ";
    pc3->Show();
    cout << pc4 << ": ";
    pc4->Show();

    delete pc2;  // free heap1
    delete pc4;  // free heap2
    delete[] buffer;  // free buffer
    cout << "Done
";

    return 0;
}

运行结果：

参考资料 

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