c++new和new()区别(了解)

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 ) with no user-declared constructors (12.1), no private or protected non-static data members (clause ), no base classes (clause ), 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  (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 ), 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 ) 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..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; }; // 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 ;
}
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; }; // 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 << 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 ;
}
运行结果：
上述测试平台是VS2008.需要注意的是，VS08只支持C++。

在这种情况下：

new A：不确定的值

new A()：zero-initialize

new B：默认构造（B::m未被初始化）

new B()：默认构造（B::m未被初始化）

new C：默认构造（C::m被zero-initialize）

new C()：默认构造( C::m被zero-initialize )

如果用兼容C++03的编译器，应该产生如下结果：

new A：不确定的值

new A()：value-initialize A，由于是POD类型所以是zero initialization

new B：默认构造（B::m未被初始化）

new B()： value-initialize B，zero-initialize所有字段，因为使用的默认构造函数

new C：default-initialize C，调用默认构造函数

new C()： value-initialize C，调用默认构造函数

在所有C++版本中，只有当A是POD类型的时候，new A和new A()才会有区别。而且，C++98和C++03会有区别。

参考资料：

What are POD types in C++?

What are Aggregates and PODs and how/why are they special?