On Wednesday, 20 November 2013 at 22:49:42 UTC, Spott wrote:
I've been screwing around with templates lately, and I'm
attempting to figure out why the following won't compile:
struct value
{
int a;
const auto
opBinary(string op, T)(in T rhs) const pure {
static if (op == "+")
return
intermediateValue!(value.plus,this,rhs)();
}
ref value opAssign(T)( in T t ) {
a = t.a;
return this;
}
static
int plus(T1, T2)(in T1 x, in T2 y) pure {
return x.a + y.a;
}
}
struct intermediateValue(alias Op, alias A, alias B)
{
auto opBinary(string op, T)(in T rhs) const pure {
static if (op == "+")
return intermediateValue!(value.plus,this,rhs)();
}
@property auto a() const pure {
return Op(A, B);
}
}
void main()
{
value a = value(2);
value b = value(3);
value c;
c = a + b;
}
The error is:
d_playground.d(34): Error: pure nested function 'a' cannot
access
mutable data 'this'
d_playground.d(34): Error: pure nested function 'a' cannot
access
mutable data 'this'
d_playground.d(10): Error: template instance
d_playground.value.opBinary!("+",
value).opBinary.intermediateValue!(plus, this, rhs) error
instantiating
d_playground.d(44): instantiated from here:
opBinary!("+",
value)
d_playground.d(44): Error: template instance
d_playground.value.opBinary!("+", value) error instantiating
What is going on? Why is 'a' not allowed to "access" mutable
data (even though it isn't modifying it)? How do I tell the
compiler to pass "this" in a const fashion?
No answer, but two notes.
First, use dpaste for such code snippets:
http://dpaste.dzfl.pl/f2f39b32
Second, what are you trying to do? intermediateValue is a struct
without members. I am not sure what 'this' means in such a case.
