is variable declaration in if statement conditional expression allowed by standard?

[Brendan]

I didn't think it was, but recently I wrote this and was surprised
when it actually worked on my compiler (G++):

if (int err_code = some_function()) {
throw std::runtime_error(strerror(err_code));
}

Testing is seemed that err_code was scoped to the if statement in the
same way it would have been in a for loop.

Is this actually allowed by the standard, or is this just a GNU
extension?

Thanks,
Brendan Miller


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[Daniel Krügler]

Am 29.05.2012 20:25, schrieb Brendan:
>
> I didn't think it was, but recently I wrote this and was surprised
> when it actually worked on my compiler (G++):
>
> if (int err_code = some_function()) {
> throw std::runtime_error(strerror(err_code));
> }
>
> Testing is seemed that err_code was scoped to the if statement in the
> same way it would have been in a for loop.
>
> Is this actually allowed by the standard, or is this just a GNU
> extension?

This is no extension, but valid C++03 and C++11. The grammar of the
if-selection statement used here is:

if ( condition ) statement

and /condition/ is defined as:

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

The second item corresponds to your example. The wording also
guarantees that the name introduced here (err_code) is local to the
if-statement.

HTH & Greetings from Bremen,

Daniel Krügler