new A 与 new A()的区别

[Arthas102]

class A
{
...//省略
}

void main()
{
A* p = NULL;
//以下这两种方式有什么区别？请用一个实际的代码例子说明，谢谢！
p = new A;
p = new A();
}

[glacier3d]

有区别吗？没有吧

[zgy1353246]

看不出区别，等待高手！！！！

[Arthas102]

[Quote=引用 1 楼 glacier3d 的回复:]
有区别吗？没有吧
[/Quote]

就是不知道有没有区别才问的阿

[giant1st]

A 是一个 Class，权且就当 int 也是个 特殊的Class为例，
p = new A; （1）
p = new A(); （2）
后面的仅仅是多了个初始化，如果有 默认构造函数，（1）就会自己初始化
（2） 是按自己的意愿 来初始化，括号中为初始化的值
new int 的话，会将p所指向的int初始化为0

[R9R9R9]

class foo
{
public:
    int val;
};



int main( void )
{


    foo* p1 = new foo();
    foo* p2 = new foo;

    cout << p1->val << endl; // val被初始化为int()，即0
    cout << p2->val << endl; // val为随机值

    return 0;
}

[R9R9R9]

具体的看C++标准的规定吧

ISO/IEC 14882:2003(E) 中 5.3.4 之 15
— If the new-initializer is omitted:
— If T is a (possibly cv-qualified) non-POD class type (or array thereof), the object is default-initialized(8.5). If T is a const-qualified type, the underlying class type shall have a user-declared default constructor.
— Otherwise, the object created has indeterminate value. If T is a const-qualified type, or a (possibly cv-qualified) POD class type (or array thereof) containing (directly or indirectly) a member of const-qualified type, the program is ill-formed;
— If the new-initializer is of the form (), the item is value-initialized (8.5);

[Arthas102]

这标准的英文还真是难以看懂啊