C++ new A 和 new A() 的区别详解

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CPP开发者   2019-7-14 05:18   3179   0
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来源:江南烟雨,2013-04-13
链接:http://blog.csdn.net/xiajun07061225/article/details/8796257
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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   
#include   
#include   
  
struct A { int m; }; // POD  
struct B { ~B(); int m; }; // non-POD, compiler generated default ctor  
struct C { C() : m() {}; ~C(); int m; }; // non-POD, default-initialising m  
  
int 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:

[quote]#include   
using namespace std;  
  
struct A { int m; }; // POD  
struct B { ~B(){}; int m; }; // non-POD, compiler generated default ctor  
struct C { C() : m() {}; ~C(){}; int m; }; // non-POD, default-initialising m  
  
int main()  
{  
    A *aObj1 = new A;  
    A *aObj2 = new A();  
    cout m
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