On Friday, 26 August 2016 at 19:51:02 UTC, Patrick Schluter 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;
}
}
getN() is not pure, simple as that (and an ideal compiler
should complain in that case). A function is pure if it depends
only of the state passed as parameter. If it touches memory
that is set outside its scope it is NOT PURE!!!
import std.stdio;
void main()
{
auto t = new Test();
writeln(t.getN()); //Prints 0
t.setN(1);
writeln(t.getN()); //Prints 1
It has to, as getN() is not pure.
}
In case you didn't get it, getN() is not pure. Marking it as
such is a BUG!.
Not according to the D spec.
