为什么`throw MyClass'不起作用而且`throw MyClass（）'呢？(Why `throw MyClass' doesn't work and

为什么`throw MyClass'不起作用而且`throw MyClass（）'呢？(Why `throw MyClass' doesn't work and `throw MyClass()' does?)

我在某处读到（现在找不到来源）

MyClass *p1 = new MyClass;

和

MyClass *p2 = new MyClass();

只要MyClass提供默认构造函数，它们本质上是等价的。 编译器理解我想要做的事情并添加空括号。

如果是这种情况，为什么我不被允许写

throw MyException;

但必须使用

throw MyException();

？ （是的，一行开头的问号。）

为了增加一些混淆， C ++ FAQ建议第二个用例（ new MyClass() ）不调用构造函数，而是调用用operator()定义的函数。

I read somewhere (cannot find the source now) that

MyClass *p1 = new MyClass;

and

MyClass *p2 = new MyClass();

are essentially equivalent, provided that MyClass provides a default constructor. The compiler understands what I want to do and adds the empty parentheses.

If this is the case, why I am not allowed to write

throw MyException;

but have to use

throw MyException();

? (Yep, a question mark at the beginning of a line.)

To add some more confusion, the C++ FAQ suggests that the second usecase (new MyClass()) does not invoke a constructor, but calls function defined with operator() instead.

最满意答案

编译器理解我想要做的事情并添加空括号。

不，不; 这两个表达式并不完全相同。 区别在于如何初始化对象：第一个使用默认初始化，而第二个使用值初始化。 因此它们对于定义默认构造函数的类是等效的; 否则，第一个将使POD对象保持未初始化，而第二个将初始化为零。

为什么我不允许写throw MyException; ？

MyException()是一个创建值初始化临时对象的表达式; 你可以抛出它就像你可以抛出任何其他合适的表达式的值。

MyException不是表达式; 它只是一个类型名称。 你只能抛出一个表达式的值，所以throw MyException; 无效。 无法创建默认初始化临时。

为了增加一些混淆，C ++ FAQ建议第二个用例（new MyClass（））不调用构造函数，而是调用用operator（）定义的函数。

不，不。 它说像List x();这样的声明List x(); 声明一个返回类型为List的函数，而不是（如人们所想）的List类型的初始化对象。 它与new-expressions或operator()无关。

The compiler understands what I want to do and adds the empty parentheses.

No it doesn't; the two expressions aren't quite equivalent. The difference is in how the objects are are initialised: the first uses default-initialisation, while the second uses value-initialisation. So they are equivalent for classes that define a default constructor; otherwise, the first will leave POD objects uninitialised, while the second will initialise them to zero.

why I am not allowed to write throw MyException;?

MyException() is an expression that creates a value-initialised temporary object; you can throw that just like you can throw the value of any other suitable expression.

MyException isn't an expression; it's just a type name. You can only throw the value of an expression, so throw MyException; is not valid. There's no way to create a default-initialised temporary.

To add some more confusion, the C++ FAQ suggests that the second usecase (new MyClass()) does not invoke a constructor, but calls function defined with operator() instead.

No it doesn't. It says that a declaration like List x(); declares a function with a return type List, not (as one might think) a value-initialised object of type List. It has nothing to do with new-expressions or operator().