为什么我不能在while循环的测试部分中添加变量声明？

问题描述

显然，你可以在一个for循环中加入一个变量声明：

  for（int i = 0; .. 。
  

我注意到你可以在if和switch语句中做同样的事情：

  if（（int i = f（））！= 0）... 
 
 switch （int ch = stream.get（））... 
  

但是当我尝试做同样的在while循环中：

  while（（int ch = stream.get（））！= -1）... 
  

编译器（VC ++ 9.0）根本不喜欢。

：我发现我可以这样做：

  while（int ch = stream.get（）！= -1）... 
  

但是由于优先规则，它被解释为：

 code> while（int ch =（stream.get（）！= -1））... 
  

$ b

解决方案

'03标准中条件的语法定义如下：

 条件：
表达式
 type-specifier-seq declarator = assignment- expression 
  

因此，上述条件只允许使用以下条件：

  if（i&& ; k）{} 
 if（（i = j）== 0）{} 
 if（int i = j）{} 
  

标准允许条件声明一个变量，然而，它们通过添加一个称为'condition'的新语法规则，可以是一个表达式或一个带初始化器的声明符。结果是，只是因为你处于的情况下，为， while 或 switch 并不意味着您可以在表达式中声明一个变量。

You can, obviously, put a variable declaration in a for loop:

for (int i = 0; ...

and I've noticed that you can do the same thing in if and switch statements as well:

if ((int i = f()) != 0) ...

switch (int ch = stream.get()) ...

But when I try to do the same thing in a while loop:

while ((int ch = stream.get()) != -1) ...

The compiler (VC++ 9.0) does not like it at all.

Is this compliant behavior? Is there a reason for it?

EDIT: I found I can do this:

while (int ch = stream.get() != -1) ...

but because of precedence rules, that's interpreted as:

while (int ch = (stream.get() != -1)) ...

which is not what I want.

解决方案

The grammar for a condition in the '03 standard is defined as follows:

condition:
  expression
  type-specifier-seq declarator = assignment-expression

The above will therefore only allow conditions such as:

if ( i && j && k ) {}
if ( (i = j) ==0 ) {}
if ( int i = j ) {}

The standard allows the condition to declare a variable, however, they have done so by adding a new grammar rule called 'condition' that can be an expression or a declarator with an initializer. The result is that just because you are in the condition of an if, for, while, or switch does not mean that you can declare a variable inside an expression.

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