c++ - Why can't I put a variable declaration in the test portion of a while loop?

Question

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.

Answer 1

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.