operator[] overloading
Author
I'm working on my free matrix library at http://www.coolgroups.com/matrixlib/, and I'm going to add this function to my matrix class:
float& operator[](int index)
{
return e[index];
}
The class currently looks like this:
struct Matrix
{
float e[16];
};
Now, my question is... let's say you have a Matrix named m.
Is it faster to access, say the 10th element, doing m.e[9] instead of doing m[9]?
I would like to do m[9] for cleanliness, but if it's a lot slower, I might as well do m.e[9].
Mike C.
http://www.coolgroups.com/
if you properly inline op[] then the compiler should (during release builds) produce the exact same code for both accesses.
Author
Thanks. Let's say I declare it like so:
inline float& operator[](int index)
{
return e[index];
}
Is that considered properly inlined?
Mike C.
http://www.coolgroups.com/
inline float& operator[](int index)
{
return e[index];
}
Is that considered properly inlined?
Mike C.
http://www.coolgroups.com/
either that or just put it inside the class like
struct foo{ void ImImplicitlyInlined(){/* do stuff */}};Quote:Original post by mike74
Thanks. Are all class functions implicitly inlined?
Mike C.
http://www.coolgroups.com/
If written inline as shown in my previous reply yes.
that is
struct foo{ void ThisWillBeInlined(){/*do stuff*/} void AndThisWillNot(); void ImExplicitilyInlined();};/* later somewhere not to far away */void foo::AndThisWillNot(){}inline void foo:ImExplicitlyInlined(){}
Author
Ahh, nice. Thanks.
One last thing. I have this output function:
ostream& operator<<(ostream& out, const Matrix &m)
{
out << m[0] << " " << m[4] << " " << m[8] << " " << m[12] << endl;
out << m[1] << " " << m[5] << " " << m[9] << " " << m[13] << endl;
out << m[2] << " " << m[6] << " " << m[10] << " " << m[14] << endl;
out << m[3] << " " << m[7] << " " << m[11] << " " << m[15];
return out;
}
However, I get this error:
c:\pointninja\matrixlib.h(45) : error C2678: binary '[' : no operator found whic
h takes a left-hand operand of type 'const Matrix' (or there is no acceptable co
nversion)
If I change the
const Matrix &m
to just
Matrix &m
it works. Is there any way to make it work while retaining the const?
Mike C.
http://www.coolgroups.com/
One last thing. I have this output function:
ostream& operator<<(ostream& out, const Matrix &m)
{
out << m[0] << " " << m[4] << " " << m[8] << " " << m[12] << endl;
out << m[1] << " " << m[5] << " " << m[9] << " " << m[13] << endl;
out << m[2] << " " << m[6] << " " << m[10] << " " << m[14] << endl;
out << m[3] << " " << m[7] << " " << m[11] << " " << m[15];
return out;
}
However, I get this error:
c:\pointninja\matrixlib.h(45) : error C2678: binary '[' : no operator found whic
h takes a left-hand operand of type 'const Matrix' (or there is no acceptable co
nversion)
If I change the
const Matrix &m
to just
Matrix &m
it works. Is there any way to make it work while retaining the const?
Mike C.
http://www.coolgroups.com/
the compiler tells you that it can't call a non-const method
on a const variable
The solution is to declare 2 method
const float & operator [] (int i) const;
and
float & operator [] (int i);
The first method let you call x=m[0]; on const matrix instance
The second let you call m[0]=3.0; but only on non-const matrix instance
on a const variable
The solution is to declare 2 method
const float & operator [] (int i) const;
and
float & operator [] (int i);
The first method let you call x=m[0]; on const matrix instance
The second let you call m[0]=3.0; but only on non-const matrix instance
provide a const version of operator [], something like:
that's a good rule of thumb anyhow if you provide op[] provide both const and non const versions of it.
struct Matrix{float e[16]; float& operator[](size_t n){ return e[n];}//returning by value instead of const& since float is a built in type //feel free to call it a pointless micro-optimization ;)float operator[](size_t n) const { return e[n];}};that's a good rule of thumb anyhow if you provide op[] provide both const and non const versions of it.
Author
Okay. It looks like it worked. On a related note, I've never seen that notation before where a keyword like const appears after the parameters of the function like this:
const float & operator [] (int i) CONST;
What exactly is that telling the compiler? And, are there other keywords that can appear in that position?
Thanks.
Mike C.
http://www.coolgroups.com/
const float & operator [] (int i) CONST;
What exactly is that telling the compiler? And, are there other keywords that can appear in that position?
Thanks.
Mike C.
http://www.coolgroups.com/
This topic is closed to new replies.
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