On Fri, 08 Oct 2010 09:26:19 -0400, Benjamin Thaut
<c...@benjamin-thaut.de> wrote:
Am 08.10.2010 11:13, schrieb Lars T. Kyllingstad:
On Fri, 08 Oct 2010 09:33:22 +0200, Benjamin Thaut wrote:
Hi, I'm writing a vec4 math struct and I have a method of which the
return value has to be a lvalue so I wonder which is the correct way to
do this:
vec4 Normalize() const { ... } //won't work, not a lvalue
ref vec4 Normalize() const {
vec4 temp;
...
return temp;
} //will this lead to a segfault or not?
The compiler shouldn't even accept this. When I try a similar thing,
DMD
says "Error: escaping reference to local variable temp".
ref vec4 Normalize() const {
vec4* temp = new vec4;
...
return *temp;
} //ugly, don't want to allocate anything on the heap
This would work, since the variable is no longer on the stack and thus
survives the return of the function.
auto ref vec4 Normalize() const {
vec4 temp;
...
return temp;
} //will this lead to a segfault?
Well, that should compile, but it doesn't work the way you want. 'auto
ref' means that the function returns by ref if the return expression is
an lvalue *and it would not be a reference to a local or a parameter*.
So for this example, your function would return by value, not by ref.
Or do I need to do it totaly in some other way?
Yes, you do. :) You are trying to create a variable on the stack, and
return it by reference. The problem is that when the function returns,
its stack frame (the memory occupied by the function's local variables)
is "released". At that point the variable doesn't exist anymore, and
any
reference to it would be invalid.
-Lars
All this was only to get it to return a lvalue. I need a lvalue to be
able to do stuff like this.
vec4 v1 = vec4(...);
vec4 v2 = vec4(...);
vec4 v3 = v1.Cross(v2.Normalize()).Normalize();
Here it complained that v2.Normalize is not a lvalue, for whatever
reason.
I'm trying to make my matrix class work even if it is const to prevent
it from coyping 16 floats everytime I pass it to a function.
mat4 opMul(ref const(mat4) m1, ref const(mat4) m2) const {
...
}
I tried many things, but it turned out that basically I have to get rid
of all the consts and let it copy the matrixes everytime.
Because either it would tell me can not call
opMul((mat4)const,(mat4)const) const with (mat4)const, (mat4)const <- I
think this is because of the ref.
Or it would tell me opMul(...) is not a lvalue if I try it to use it
like this
vec4 v1,v2;
mat4 m;
vec4 res = (v1 - m * v2);
I'm coming form C++ so is it the correct way to overload such operators
without const at all? Because obviously I don't want it to copy
unneccessarily.
The correct way is to use auto ref as the parameter:
struct vec4
{
...
vec4 Normalize(auto ref const(vec4) param) {...}
}
But AFAIK, this doesn't really work.
What it *should* do is generate two versions of Normalize, one which
passes param by reference for lvalues, and one that passes by value for
rvalues. Passing rvalues by value is more efficient and less problematic
than passing by reference, since the value is already located on the
stack, and there is no need to make a copy.
-Steve
