C++ 没有参数的构造函数上没有括号是语言标准吗？
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Is no parentheses on a constructor with no arguments a language standard?
提问by mring
I was compiling a C++ program in Cygwin using g++ and I had a class whose constructor had no arguments. I had the lines:
我正在使用 g++ 在 Cygwin 中编译一个 C++ 程序，我有一个类的构造函数没有参数。我有以下几行：
MyClass myObj();
myObj.function1();
And when trying to compile it, I got the message:
在尝试编译它时，我收到了消息：
error: request for member 'function1' in 'myObj', which is of non-class type 'MyClass ()()'
After a little research, I found that the fix was to change that first line to
经过一番研究，我发现修复方法是将第一行更改为
MyClass myObj;
I could swear I've done empty constructor declarations with parentheses in C++ before. Is this probably a limitation of the compiler I'm using or does the language standard really say don't use parentheses for a constructor without arguments?
我可以发誓我以前在 C++ 中做过带括号的空构造函数声明。这可能是我使用的编译器的限制，还是语言标准真的说不要在没有参数的构造函数中使用括号？
回答by CB Bailey
Although MyClass myObj();could be parsed as an object definition with an empty initializer or a function declaration the language standard specifies that the ambiguity is always resolved in favour of the function declaration. An empty parentheses initializer is allowed in other contexts e.g. in a newexpression or constructing a value-initializedtemporary.
尽管MyClass myObj();可以将其解析为具有空初始值设定项或函数声明的对象定义，但语言标准指定始终有利于函数声明来解决歧义。在其他上下文中允许使用空括号初始值设定项，例如在new表达式中或构造值初始化的临时值。
回答by Peter Alexander
This is called the most vexing parse issue. When the parser sees
这被称为最棘手的解析问题。当解析器看到
MyClass myObj();
It thinks you are declaring a function called myObjthat has no parameters and returns a MyClass.
它认为您正在声明一个myObj没有参数的调用函数并返回一个MyClass.
To get around it, use:
要绕过它，请使用：
MyClass myObj;
回答by suszterpatt
I found this in the C++ standard (§8.5.8):
我在 C++ 标准（第 8.5.8 节）中发现了这一点：
An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.
[Note: since () is not permitted by the syntax for initializer,
X a ();is not the declaration of an object of class X, but the declaration of a function taking no argument and returning an X. The form () is permitted in certain other initialization contexts (5.3.4, 5.2.3, 12.6.2). —end note ]
初始值设定项为空括号集的对象，即 ()，应进行值初始化。
[注意：由于 () 不允许用于初始化程序的语法，
X a ();不是类 X 的对象的声明，而是不带参数并返回 X 的函数的声明。形式 () 在某些其他初始化上下文中是允许的（5.3.4、5.2.3、12.6.2）。——尾注]
回答by Liz Albin
This is a fairly well known issue, and isn't compiler dependent. Essentially, what you were doing was declaring a function returning type MyObj. Not surprisingly, you couldn't call its constructor. See the C++ faq litefor a good explanation
这是一个众所周知的问题，与编译器无关。本质上，您所做的是声明一个返回类型 MyObj 的函数。毫不奇怪，你不能调用它的构造函数。请参阅C++ faq lite以获得很好的解释
回答by sbk
MyClass myObj();
That's parsed as a function declaration, the function is called myObj, takes no arguments and returns MyClass object. I've never seen a compiler accepting that. On the other hand MyClass* myPtr = new MyClass();is acceptable, may be that got you confused?
这被解析为函数声明，该函数称为 myObj，不接受任何参数并返回 MyClass 对象。我从未见过编译器接受这一点。另一方面MyClass* myPtr = new MyClass();是可以接受的，可能会让你感到困惑吗？
回答by AProgrammer
Yet another most-vexing-parse hit. See for instance Sort function does not work with function object created on stack?
另一个最令人烦恼的解析命中。请参阅例如Sort 函数不适用于在堆栈上创建的函数对象？
回答by Ari
Your line makes the compiler think you are declaring a function named myObjwhich takes no arguments and returns a MyClass. This ambiguity resolution is indeed annoying.
您的行使编译器认为您正在声明一个名为的函数myObj，该函数不接受任何参数并返回一个MyClass. 这种歧义解析确实很烦人。
回答by JonH
The standard does not require parentheses.
该标准不需要括号。
int* x = new int;
is legal syntax.
是合法的语法。
In your case myclass myobj();is a function prototype. Whereas myclass myobj;is a variable.
在你的情况下myclass myobj();是一个函数原型。而myclass myobj;是一个变量。
