为什么C ++允许我们在声明变量时在括号中包围变量名？ [英] Why does C++ allow us to surround the variable name in parentheses when declaring a variable?

问题描述

例如，一个声明如：

int (x) = 0;

或甚至：

int (((x))) = 0;

我偶然发现这是因为在我的代码中我碰巧有一个类似于下面的片段： / p>

I stumbled upon this because in my code I happened to have a fragment similar to the following one:

struct B
{
};

struct C
{
  C (B *) {}
  void f () {};
};

int main()
{
  B *y;
  C (y);
}

显然我想构造对象 C 然后会做一些有用的在它的析构函数。然而，因为它发生编译器处理 C（y）; 作为变量 y > C ，因此它会输出一个关于 y 重定义的错误。有趣的是，如果我将它写为 C（y）.f（）或像 C（static_cast< B *> ））它会按预期进行编译。最好的现代解决方法是在构造函数调用中使用 {} 。

Obviously I wanted to construct object C which then would do something useful in its destructor. However as it happens compiler treats C (y); as a declaration of variable y with type C and thus it prints an error about y redefinition. Interesting thing is that if I write it as C (y).f () or as something like C (static_cast<B*> (y)) it will compile as intended. The best modern workaround is to use {} in constructor call, of course.

可以声明如 int（x）= 0; 或甚至 int（（x）））= 0; 但我从来没有见过任何人实际使用这样的声明。所以我感兴趣 - 这是这样的可能性的目的，因为现在我看到它只创建类似臭名昭着的最烦琐的解析的情况，不添加任何有用的？

So as I figured out after that, it's possible to declare variables like int (x) = 0; or even int (((x))) = 0; but I've never seen anyone actually using declarations like this. So I'm interested -what's the purpose of such possibility because for now I see that it only creates the case similar to the notorious "most vexing parse" and doesn't add anything useful?

推荐答案

分组。

作为一个特定的例子，你可以声明一个函数类型的变量， / p>

As a particular example, consider that you can declare a variable of function type such as

int f(int);

现在，你如何声明一个指向这样的东西的指针？

Now, how would you declare a pointer to such a thing?

int *f(int);

不，不行！这被解释为返回 int * 的函数。您需要添加括号以使其按正确的方式解析：

Nope, doesn't work! This is interpreted as a function returning int*. You need to add in the parentheses to make it parse the right way:

int (*f)(int);

与数组相同的处理：

int *x[5];   // array of five int*
int (*x)[5]; // pointer to array of five int

这篇关于为什么C ++允许我们在声明变量时在括号中包围变量名？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！