【C++】new A和new A()的区别详解

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我们在C++程序中经常看到两种new的使用方式：new A以及new A()。那么这两种究竟有什么区别呢？

调用new分配的内存有时候会被初始化，而有时候不会，这依赖于A的类型是否是POD（Plain old data）类型，或者它是否是包含POD成员、使用编译器生成默认构造函数的类。

附：POD类型

POD是Plain old data的缩写，它是一个struct或者类，且不包含构造函数、析构函数以及虚函数。

维基百科给出了更加详细的解释：

C++的POD类型或者是一个标量值，或者是一个POD类型的类。POD class没有用户定义的析构函数、拷贝构造函数和非静态的非POD类型的数据成员。而且，POD class必须是一个aggregate，没有用户定义的构造函数，没有私有的或者保护的非静态数据，没有基类或虚函数。它只是一些字段值的集合，没有使用任何封装以及多态特性。

附：aggregate的定义：

An aggregate is an array or a class (clause 9) with no user-declared constructors (12.1), no private or protected non-static data members (clause 11), no base classes (clause 10), and no virtual functions (10.3).

接着介绍一下C++中的三种初始化方式：

zero-initialization,default-initialization,value-initialization。

首先需要注意的是value-initialization是在C++2003标准中新引入的，在原来的1998标准中并不存在。

C++03标准中针对这三种方式的说明：

To zero-initialize an object of type T means:
— if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T;
— if T is a non-union class type, each nonstatic data member and each base-class subobject is zero-initialized;
— if T is a union type, the object’s first named data member is zero-initialized;
— if T is an array type, each element is zero-initialized;
— if T is a reference type, no initialization is performed.

To default-initialize an object of type T means:
— if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is an array type, each element is default-initialized;
— otherwise, the object is zero-initialized.

To value-initialize an object of type T means:
— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized

A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed. If T is a cv-qualified type, the cv-unqualified version of T is used for these definitions of zero-initialization, default-initialization, and value-initialization.

注意：VS2008遵循的是98标准，而GCC3.4.5遵循的是03标准。

采用如下代码可以验证编译器遵循的到底是哪一种标准：

#include <stdio.h>#include <string.h>#include <new>struct A { int m; }; // PODstruct B { ~B(); int m; }; // non-POD, compiler generated default ctorstruct C { C() : m() {}; ~C(); int m; }; // non-POD, default-initialising mint main(){    char buf[sizeof(B)];    memset( buf, 0x5a, sizeof( buf));    // use placement new on the memset'ed buffer to make sure     //  if we see a zero result it's due to an explicit     //  value initialization    B* pB = new(buf) B();   //C++98 rules - pB->m is uninitialized                            //C++03 rules - pB->m is set to 0    printf( "m  is %d\n", pB->m);    return 0;}

在VS008中输出就不是0，说明遵循的是98标准。

下面先看一段C++示例代码：

#include <iostream>using namespace std;struct A { int m; }; // PODstruct B { ~B(){}; int m; }; // non-POD, compiler generated default ctorstruct C { C() : m() {}; ~C(){}; int m; }; // non-POD, default-initialising mint main(){A *aObj1 = new A;A *aObj2 = new A();cout << aObj1->m << endl;cout << aObj2->m << endl;B *bObj1 = new B;B *bObj2 = new B();cout << bObj1->m << endl;cout << bObj2->m << endl;C *cObj1 = new C;C *cObj2 = new C();cout << cObj1->m << endl;cout << cObj2->m << endl;delete aObj1;delete aObj2;delete bObj1;delete bObj2;delete cObj1;delete cObj2;return 0;}

运行结果：