Whats the error with this c++ construct?

Question

I am a newbee in C++. Tried the following code:

while((char c = cin.get()) != 'q')
{  //do anything
}

when I try to compile, it fails with following

error: expected primary-expression before "char".

Please help me understand this

Answer 1

You cannot have a declaration as a part of an expression.

while ((char c = cin.get()) != 'q') { ...
//      |----------------| <---------------------- this is a declaration
//     |-------------------------| <-------------- this is an expression

You can have a declaration directly inside the parentheses of the loop (not in any nested parentheses):

while (char c = cin.get()) { ...

but this stops on !c, which is not what you want.

This will work:

while (int c = cin.get() - 'q') { // ugly code for illustrative purpose
 c += 'q';
 ...
}

and so will this:

for (char c; (c = cin.get()) != 'q'; ) { // ugly code for illustrative purpose
  ...
}

Update: see also this SO question.

Answer 2

Try this:

char c;
while((c = cin.get()) != 'q')
{  //do anything
}

You are declaring the variable inside parantheses, hence the error:

while (char c = cin.get() != 'q')