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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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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;
}
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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