new、operator new 、placement new

代码来自 www.cplusplus.com

直接上代码

#include      // std::cout
#include           // ::operator newstruct MyClass {int data[100];MyClass() {std::cout << "constructed [" << this << "]\n";}
};int main () {//newstd::cout << "1: ";MyClass * p1 = new MyClass;// allocates memory by calling: operator new (sizeof(MyClass))// and then constructs an object at the newly allocated space//调用 operator new 分配内存，然后调用构造函数生成一个对象，并返回指针std::cout << "2: ";MyClass * p2 = new (std::nothrow) MyClass;// allocates memory by calling: operator new (sizeof(MyClass),std::nothrow)// and then constructs an object at the newly allocated space//placement newstd::cout << "3: ";new (p2) MyClass;// does not allocate memory -- calls: operator new (sizeof(MyClass),p2)// but constructs an object at p2//不分配内存，在p2指向的内存上重新构造一个对象//operator new// Notice though that calling this function directly does not construct an object:std::cout << "4: ";MyClass * p3 = (MyClass*) ::operator new (sizeof(MyClass));// allocates memory by calling: operator new (sizeof(MyClass))// but does not call MyClass's constructor//分配内存，但是不调用构造函数
//placement newint a=100;cout<<"before:"<<a<<endl;int *ptr=new (&a) int(50);cout<<"after:"<<a<<endl;delete p1;delete p2;delete p3;return 0;
}

代码结果

1: constructed [0x8f0f70]
2: constructed [0x8f23a8]
3: constructed [0x8f23a8]
4: 
before:100
after:50

总结

• new 调用operator new分配内存，然后调用构造函数，最后返回指针
• operator new 只分配内存，不调用构造函数
• placement new 是 operator new 的重载，不分配内存，在已有内存上重新构造一个对象

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