在初始化使用新声明的变量（INT X = X + 1）？ [英] Using newly declared variable in initialization (int x = x+1)?

问题描述

我只是偶然发现了一个行为，让我吃惊：

当写：

  INT X = X + 1;
 

在C / C ++ - 程序（或涉及新创建的变量x更加复杂的前pression）我的gcc / g ++的编译没有错误。在上述情况下，X是1之后。需要注意的是在范围上没有变量x由previous声明。

所以我想知道这是否是正确的行为（甚至可能在某些情况下非常有用），与一般的我的gcc版本或GCC或只是一个解析器特征模拟。

BTW：下不工作：

  INT X ++;
 

解决方案

随着前pression：

  INT X = X + 1;
 

变量 X 进入在 = 的标志，这就是为什么你可以使用它在正确的存在右手边。通过开始存在，我指的是变量存在，但尚未被初始化器部分被分配一个值。

不过，除非你初始化静态存储持续时间（例如，函数外）一个变量，它是不确定的行为，因为 X 即开始存在有任意值。

C ++ 03有这样一段话：

  

申报为名称的点是立即其完整的声明符（第8条）后，和它的初始化之前（如果有的话）...

  
  

例如：结果
   INT X = 12; 结果
   {INT X = X; } 结果
  在这里，第二个X初始化它自己（不确定）值。

这是第二个案例有pretty多少你有你的问题。

I just stumbled upon a behavior which surprised me:

When writing:

int x = x+1;

in a C/C++-program (or even more complex expression involving the newly created variable x) my gcc/g++ compiles without errors. In the above case X is 1 afterwards. Note that there is no variable x in scope by a previous declaration.

So I'd like to know whether this is correct behaviour (and even might be useful in some situation) or just a parser pecularity with my gcc version or gcc in general.

BTW: The following does not work:

int x++;

解决方案

With the expression:

int x = x + 1;

the variable x comes into existence at the = sign, which is why you can use it on the right hand side. By "comes into existence", I mean the variable exists but has yet to be assigned a value by the initialiser part.

However, unless you're initialising a variable with static storage duration (e.g., outside of a function), it's undefined behaviour since the x that comes into existence has an arbitrary value.

C++03 has this to say:

The point of declaration for a name is immediately after its complete declarator (clause 8) and before its initializer (if any) ...

Example:
int x = 12;
{ int x = x; }
Here the second x is initialized with its own (indeterminate) value.

That second case there is pretty much what you have in your question.

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