On Wednesday, 19 July 2017 at 15:39:02 UTC, Steven Schveighoffer
wrote:
On 7/19/17 9:30 AM, sontung wrote:
So I was thinking of a way of extending if statements that
have declarations. The following being as example of the
current use of if statements with declarations:
if(int* weDontPollute = someFunc())
{
// use weDontPollute
}
That's great and all, but it only works by checking if the
variable evaluates to true or false. Which is fine for a
pointer but otherwise useless for anything else, like integers
where zero is usually valid input (index for array). So
currently the only way to do something like this in the
language, that i've found, is to use a for statement.
for(int i = someFunc(); i >= 0;)
{
// use i
break;
}
Not that ideal to use a for statement. It makes it hard to
read and if the break condition isn't there it might very well
be an infinite loop. So I was thinking of some sort of syntax
like this:
if(int i = someFunc(); i >= 0)
{
// use i
}
Thoughts on this sort of feature?
I really like the idea. Only thing I don't like is the
possibility for abuse/confusion/errors:
if(int i = someFunc(); j >= 0) // typo, or weird relationship,
or just intentional obfuscation?
It reminds me a bit of why we got rid of the comma operator.
This is why I've liked suggestions in the past like:
if((int i = foo()) >= 0)
That is, you want to use 'if' on an expression while saving the
expression, but the if is only looking at a property of that
expression.
Note this makes if(arr) (the correct meaning, that is ;) much
more palatable:
if((auto x = getArray()).length)
Don't get me wrong, if this syntax is what gets this idea in,
I'm fine with it.
One possibility is to require usage of the declared variable in
the condition.
-Steve
I respectfully disagree with this. I recall many times where I
want to declare variables that should be limited to the scope of
the conditional block but aren't used in the condition itself, i.e
{
auto a = ...;
auto b = ...;
while(something)
{
// use a and b
}
}
I imagine this feature allowing it to be rewritten as:
while(auto a = ...;
auto b = ...;
something)
{
// use a and b
}
