The following code assert fails (bar == 1, not -10!). I've wasted a bit of time because of this happening.

void main() {
	if (int bar = foo() != 0) {
		assert(bar == -10);
	}
}

auto foo() {
	return -10;
}

On Tuesday, October 31, 2017 04:08:12 Joel via Digitalmars-d-learn wrote:
> The following code assert fails (bar == 1, not -10!). I've wasted a bit of time because of this happening.
>
> void main() {
>   if (int bar = foo() != 0) {
>       assert(bar == -10);
>   }
> }
>
> auto foo() {
>   return -10;
> }

Why would you expect it to be -10? bar is assigned the result of foo() != 0, which is a boolean result and will be either true or false. When that's assigned to int, a conversion occurs, and that conversion treats true as 1 and false as 0. If you want the result to be -10, then you need to assign the result of foo() to bar, not the result of foo() != 0. Now, because 0 is treated as false, you could probably still do

if(int bar = foo())

rather than something like

int bar = foo();
if(bar != 0)

but regardless, the result of foo() != 0 is going to be bool, not int, so all you'll ever get out of it is true or false, which will then be 1 or 0 if converted to int.

- Jonathan M Davis

On Tuesday, 31 October 2017 at 04:08:12 UTC, Joel wrote:
> 	if (int bar = foo() != 0) {

Not a bug, but I do think it is an iffy design.

That is more like:

int bar;
if(bar = (foo() != 0))

so the foo != 0 is evaluated first, which ends up being boolean true or false, then THAT true/false value is converted to int and assigned to bar, hence it becomes 0 or 1.


You can't really combine declaration and non-trivial comparison in a single statement.

Ok, thanks guys.

On Tuesday, 31 October 2017 at 04:27:27 UTC, Joel wrote:
> Ok, thanks guys.

why not throw in some UFCS too...just because you can ;-)

import std.stdio;

void main()
{
    int foo;
    if (foo.bar != 0)  // would be nice if I could do: (int foo.bar != 0)
    {
        throw new Exception("foo.bar != 0");
    }
}

auto bar(int x,)
{
    return -10;
}