重载new和delete运算符

时间：2023-03-09 00:37:12
重载new和delete运算符

内存管理运算符 new、new[]、delete 和 delete[] 也可以进行重载，其重载形式既可以是类的成员函数，也可以是全局函数。一般情况下，内建的内存管理运算符就够用了，只有在需要自己管理内存时才会重载。

以成员函数的形式重载 new 运算符：

void * className::operator new( size_t size ){

    //TODO:

}

以全局函数的形式重载 new 运算符：

void * operator new( size_t size ){

    //TODO:

}

两种重载形式的返回值相同，都是void *类型，并且都有一个参数，为size_t类型。

在重载 new 或 new[] 时，无论是作为成员函数还是作为全局函数，它的第一个参数必须是 size_t 类型。size_t 表示的是要分配空间的大小，对于 new[] 的重载函数而言，size_t 则表示所需要分配的所有空间的总和。

重载的new必须有一个size_t参数。这个参数由编译器产生并传递给我们，它是要分配内存的对象的长度。必须返回一个指向等于这个长度（或者大于这个长度）的对象的指针。如果没有找到存储单元（在这种情况下，构造函数不被调用），则返回一个0。然后如果找不到存储单元，不能仅仅返回0，我们还应该调用new-handler或产生一个异常信息之类的事，告诉这里出现了问题。

我们首先需要明白的一点就是：operator new()的返回值是一个void*，而不是指向任何特定类型的指针。所做的是分配内存，而不是完成一个对象建立--直到构造函数调用了才完成对象的创建，它是编译器确保做的动作，不在我们的控制范围之内了，所以我们就没有必要考虑。

size_t 在头文件 <cstdio> 中被定义为typedef unsigned int size_t;，也就是无符号整型。

当然，重载函数也可以有其他参数，但都必须有默认值，并且第一个参数的类型必须是 size_t。

同样的，delete 运算符也有两种重载形式。以类的成员函数的形式进行重载：

void className::operator delete( void *ptr){

    //TODO:

}

以全局函数的形式进行重载：

void operator delete( void *ptr){

    //TODO:

}

两种重载形式的返回值都是 void 类型，并且都必须有一个 void 类型的指针作为参数，该指针指向需要释放的内存空间。

当我们以类的成员函数的形式重载了new 和 delete 操作符，其使用方法如下：

C * c = new C; //分配内存空间

//TODO:

delete c; //释放内存空间

如果类中没有定义 new 和 delete 的重载函数，那么会自动调用内建的 new 和 delete 运算符。

重载全局new和delete

#include <QCoreApplication>

#include<iostream>

#include<cstdio>

#include<cstring>

#include<cstdlib>

#include<cmath>

#include<algorithm>

#define inf 0x7fffffff

using namespace std;

void* operator new(size_t sz)

{

    printf("operator new: %d Bytes\n",sz);

    void* m = malloc(sz);

    if (!m) puts("out of memory");

    return m;

}

void* operator new [](size_t sz)

{

    printf("operator new: %d Bytes\n",sz);

    void* m = malloc(sz);

    if (!m) puts("out of memory");

    return m;

}

void operator delete(void* m)

{

    puts("operator delete");

    free(m);

}

void operator delete[](void* m)

{

    puts("operator delete");

    free(m);

}

class S

{

public:

    S() {puts("S::S()"); }

    ~S() {puts("S::~S()"); }

private:

    int an[];

};

void func()

{

    puts("creating & destroying an int");

    int* q = new int();

    delete q;

    puts("creating & destroying an int[]");

    int* p = new int[]();

    delete []p;

    puts("creating & destroying an s");

    S* s = new S;

    delete s;

    puts("creating & destroying S[3]");

    S* sa = new S[];

    delete []sa;

}

int main(int argc, char *argv[])

{

    QCoreApplication a(argc, argv);

    func();

    return a.exec();

}

输出：

creating & destroying an int

operator new:  Bytes

operator delete

creating & destroying an int[]

operator new:  Bytes

operator delete

creating & destroying an s

operator new:  Bytes

S::S()

S::~S()

operator delete

creating & destroying S[]

operator new:  Bytes

S::S()

S::S()

S::S()

S::~S()

S::~S()

S::~S()

operator delete

对于一个类重载new和delete

为一个类重载new和delete的时候，尽管不必显式的使用static，但是实际上仍是在创建static成员函数。它的语法也和重载任何其它运算符一样。当编译器看到使用new创建自己定义的类的对象时，它选择成员版本的operator new()而不是全局版本的new()。但是全局版本的new和delete仍为所有其他类型对象使用（除非它们也有自己的new和delete）。这个和全局变量、局部变量的意思是一样的。

#include<iostream>

#include<cstddef>

#include<fstream>

#include<new>

using namespace std;

ofstream out("Framis.out");

class Framis

{

public:

    enum{psize =  };

    Framis() {out<< "Framis()" <<endl; }

    ~Framis() {out<< "~Framis() ... " <<endl; }

    void* operator new(size_t) throw (bad_alloc);

    void operator delete(void*);

private:

    enum{sz =  };

    char c[sz];

    static unsigned char pool[];

    static bool alloc_map[];

};

unsigned char Framis::pool[psize*sizeof(Framis)];

bool Framis::alloc_map[psize]={false};

void* Framis::operator new(size_t sz) throw(bad_alloc)

{

    for (int i=; i<psize; ++i) {

        if (!alloc_map[i]) {

            out<< "using block " << i << " ... ";

            alloc_map[i]=true;

            return pool+(i*sizeof(Framis));

        }

    }

    out<< "out of memory" <<endl;

    throw bad_alloc();

}

void Framis::operator delete(void* m)

{

    if (!m) return;

    unsigned long block = (unsigned long)m-(unsigned long)pool;

    block /= sizeof(Framis);

    out<< "freeing block " << block <<endl;

    alloc_map[block]=false;

}

int main()

{
    cout << sizeof(Framis) << endl;

    Framis* f[Framis::psize];

    try {

        for (int i=; i<Framis::psize; i++) {

            f[i]=new Framis; // new 的时候先跑进 new 重载函数中，再调用构造函数

        }

        new Framis;

    } catch(bad_alloc) {

        cerr<< "Out of memory!" <<endl;

    }

    delete f[];

    f[]=;

    Framis* X=new Framis;

    delete X;// delete 的时候先调用析构函数，再跑进delete重载函数中
for (int j=; j<Framis::psize; j++) {

        delete f[j];

    }

    return ;

}
 // sizeof Framis

using block  ... Framis()

using block  ... Framis()

using block  ... Framis()

using block  ... Framis()

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out of memory

Out of memory!

~Framis() ...

freeing block

using block  ... Framis()

~Framis() ...

freeing block

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freeing block

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freeing block

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freeing block

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输出

为数组重载new和delete

上一段文字中我们讲到如果为一个类重载operator new()和operator delete()，那么无论何时创建这个类的一个对象都将调用这些运算符。但是如果要创建这个类的一个对象数组的时候，全局operator new()就会被立即调用，用来为这个数组分配足够的内存。对此，我们可以通过为这个类重载运算符的数组版本，即operator new[]和operator delete[]，来控制对象数组的内存分配。

#include<iostream>

#include<fstream>

#include<new>

using namespace std;

ofstream trace("ArrayOperatorNew.out");

class Widget

{

public:

    Widget() {trace<< "*" <<endl; }

    ~Widget() {trace<< "~" <<endl; }

    void* operator new(size_t sz) {

        trace<< "Widget::new: " << sz << " byte" <<endl;

        return ::new char[sz];

    }

    void operator delete(void* p) {

        trace<< "Widget::delete" <<endl;

        ::delete []p;

    }

    void* operator new[](size_t sz) {

        trace<< "Widget::new[]: " << sz << " bytes" <<endl;

        return ::new char[sz];

    }

    void operator delete[](void* p) {

        trace<< "Widget::delete[]" <<endl;

        ::delete []p;

    }

private:

    enum{sz= };

    int an[sz];

};

int main()

{

    trace<< "new Widget" <<endl;

    Widget* w=new Widget;

    trace<<endl<< "delete Widget" <<endl;

    delete w;

    trace<<endl<< "new Widget[25]" <<endl;

    Widget* wa=new Widget[];

    trace<<endl<< "delete []Widget" <<endl;

    delete []wa;

    return ;

}
new Widget

Widget::new:  byte

*

delete Widget

~

Widget::delete

new Widget[]

Widget::new[]:  bytes

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

delete []Widget

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

~

Widget::delete[]

输出

C++空类的大小是1

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