C++ 带空括号的默认构造函数
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/180172/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Default constructor with empty brackets
提问by Martin Beckett
Is there any good reason that an empty set of round brackets (parentheses) isn't valid for calling the default constructor in C++?
在 C++ 中，一组空的圆括号（圆括号）对于调用默认构造函数无效，是否有充分的理由？
MyObject object; // ok - default ctor
MyObject object(blah); // ok
MyObject object(); // error
I seem to type "()" automatically everytime. Is there a good reason this isn't allowed?
我似乎每次都会自动输入“（）”。有充分的理由不允许这样做吗？
采纳答案by Constantin
Most vexing parse
最烦人的解析
This is related to what is known as "C++'s most vexing parse". Basically, anything that can be interpreted by the compiler as a function declaration will be interpreted as a function declaration.
这与所谓的“C++ 最烦人的解析”有关。基本上，任何可以被编译器解释为函数声明的东西都将被解释为函数声明。
Another instance of the same problem:
同样问题的另一个例子：
std::ifstream ifs("file.txt");
std::vector<T> v(std::istream_iterator<T>(ifs), std::istream_iterator<T>());
vis interpreted as a declaration of function with 2 parameters.
v被解释为带有 2 个参数的函数声明。
The workaround is to add another pair of parentheses:
解决方法是添加另一对括号：
std::vector<T> v((std::istream_iterator<T>(ifs)), std::istream_iterator<T>());
Or, if you have C++11 and list-initialization (also known as uniform initialization) available:
或者，如果您有 C++11 和列表初始化（也称为统一初始化）可用：
std::vector<T> v{std::istream_iterator<T>{ifs}, std::istream_iterator<T>{}};
With this, there is no way it could be interpreted as a function declaration.
有了这个，就无法将其解释为函数声明。
回答by 1800 INFORMATION
Because it is treated as the declaration for a function:
因为它被视为函数的声明：
int MyFunction(); // clearly a function
MyObject object(); // also a function declaration
回答by Nemanja Trifunovic
The same syntax is used for function declaration - e.g. the function object, taking no parameters and returning MyObject
相同的语法用于函数声明 - 例如函数object，不带参数并返回MyObject
回答by Fred Larson
Because the compiler thinks it is a declaration of a function that takes no arguments and returns a MyObject instance.
因为编译器认为它是一个不带参数并返回一个 MyObject 实例的函数声明。
回答by dalle
You could also use the more verbose way of construction:
您还可以使用更详细的构造方式：
MyObject object1 = MyObject();
MyObject object2 = MyObject(object1);
In C++0x this also allows for auto:
在 C++0x 中，这也允许auto：
auto object1 = MyObject();
auto object2 = MyObject(object1);
回答by Black
I guess, the compiler would not know if this statement:
我猜，编译器不知道这个语句是否：
MyObject object();
MyObject 对象();
is a constructor call or a function prototype declaring a function named objectwith return type MyObjectand no parameters.
是构造函数调用或函数原型，声明一个名为object的函数，返回类型为MyObject且没有参数。
回答by Michael Burr
As mentioned many times, it's a declaration. It's that way for backward compatibility. One of the many areas of C++ that are goofy/inconsistent/painful/bogus because of its legacy.
正如多次提到的，这是一个声明。这是向后兼容的方式。由于其遗留问题，C++ 的众多领域之一是愚蠢的/不一致的/痛苦的/虚假的。
回答by Andreas DM
From n4296 [dcl.init]:
来自 n4296 [dcl.init]：
[ Note:
Since()is not permitted by the syntax for initializer,X a();is not the declaration of an objectof class X, but the declaration of a functiontaking 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 ]
[注意：
因为初始化器()的语法不允许， 所以不是类 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]X a();
回答by Hitokage
As the others said, it is a function declaration. Since C++11 you can use brace initialization if you need to see the empty somethingthat explicitly tells you that a default constructor is used.
正如其他人所说，它是一个函数声明。从 C++11 开始，如果您需要查看明确告诉您使用默认构造函数的空东西，则可以使用大括号初始化。
Jedi luke{}; //default constructor
