On Friday, 26 August 2016 at 14:12:24 UTC, Basile B. wrote:
On Friday, 26 August 2016 at 14:03:13 UTC, Meta wrote:
On Friday, 26 August 2016 at 10:51:15 UTC, Johan Engelen wrote:
On Thursday, 25 August 2016 at 14:42:28 UTC, Basile B. wrote:
I'll add
* create temporaries based on the const function attribute.
Struct method constness (as in your example) does not mean
that the return value is constant when calling it twice in a
row. As pointed out by others, the function needs to be pure.
Dlang pureness is not a strong enough guarantee.
For example, this is explicitly allowed by the spec:
```
pure int foo()
{
debug writeln("in foo()"); // ok, impure code allowed in
debug statement
return 1;
}
```
That makes it illegal to transform `foo()+foo()` to `a=foo();
a+a`, at least in debug builds.
David discusses your proposed optimization, and why it cannot
be done in general (!) on Dlang pure functions.
http://klickverbot.at/blog/2012/05/purity-in-d/
-Johan
Here's an example that doesn't even need to use debug
statements, and is perfectly legal.
class Test
{
int n;
void setN(int val) pure
{
n = val;
}
int getN() const pure
{
return n;
}
}
import std.stdio;
void main()
{
auto t = new Test();
writeln(t.getN()); //Prints 0
t.setN(1);
writeln(t.getN()); //Prints 1
}
That's another story. Of course the optimization pass for this
should check that **ALL** the calls to Test in a sub program
(or in this scope if you want) are const... Which is not the
case here.
void foo(Bar bar)
{
foreach(i; 0 .. bar.length) // bar.length can be evaluated
once
{
stuffA.a = bar[i].propertyA // bar[i] can be cached
stuffB.b = bar[i].propertyB // to get propertyB
}
}
with
interface Bar
{
size_t length() pure const;
Stuff opIndex(size_t) pure const;
}
