Re: declaration of a char in an if-statement fails to comile

From:
"kanze" <kanze@gabi-soft.fr>
Newsgroups:
comp.lang.c++.moderated
Date:
31 May 2006 16:42:52 -0400
Message-ID:
<1149065133.063666.92780@i39g2000cwa.googlegroups.com>
Victor Bazarov wrote:

pavel.lukin@gmail.com wrote:

the problem is briefly described in the subjec, so I'll just
give the sample code:

<code>
#include <string>

int main()
{

  std::string fileName("filename.txt");
  int ix = 8;

  if(( char c = fileName[ix] ) == '.' )
  {
    c = '_';
    fileName[ix] = c;
  }
}
</code>

I tried to compile this with g++, MS Visual Studio Express 2005 and
comeau, neither succeeded.
Am I missing something here? A clear explanation will be greatly
appreciated.


Yes, you're missing the simple fact that a declaration is
allowed between the 'if's parentheses, but not in an arbitrary
expression. Since you've surrounded the declaration with its
own set of parentheses, the syntax you're trying to use is not
allowed. You could rewrite this as

     if (char c = !(fileName[ix] - '.'))


Presumably, he was trying to capture fileName[ix] in c, not a
boolean. And your rewrite looks like a real candidate for an
obfuscated code contest -- if you really do want to capture in c
the fact that fileName[ix] is equal to '.', the obvious way to
do it would be:

    if ( char c = (fileName[ix] == '.') )

This has exactly the same semantics as what you wrote, but is
considerably more readable.

but there is no need. Since you're not using the value of 'c'
with which it is initialised, the right way would be:

     if (fileName[ix] == '.')
         fileName[ix] = '_';

And you do away with 'c' altogether.


Certainly. I think he just knocked out a quick and dirty
example of the syntax problem he was encountered.

What is allowed, for example is this:

    if (ifstream myfile(filename)) {
        // do something with 'myfile'
    }

which limits the scope of 'myfile' to that 'if' statement and
its controlled statement,


Including any else part, so the only thing you've gained is a
bit of obfuscation.

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James Kanze GABI Software
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