C++ new A 和 new A() 的区别详解

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 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:

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:

运行结果:

上述测试平台是VS2008.需要注意的是,VS08只支持C++98。

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,比如G++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?

1 3 收藏 评论

相关文章

可能感兴趣的话题



直接登录
跳到底部
返回顶部