Access class members by its string label
KK
Please check the below mentioned class. I am looking for a smarter way
to define the function 'getValueByLabelMatch'. getValueByLabelMatch
takes a variable label and retrieves the value of the respective class
member. For simplicity I have used members of int type, but they could
be of any data type.
/* variable scope deliberately given public scope */
#include <iostream>
using namespace std;
class someclass
{
public:
int nApples, nOranges, nGrapes;
someclass(int a,int b, int c):nApples(a),nOranges(b),nGrapes(c){}
int getValueByLabelMatch(string givestr)
{
if (givestr.compare("nApples")==0 )
return nApples;
//other if conditions for each of the member variables
return 0;
}
};
void main()
{
someclass tst(1,2,3);
int a = tst.getValueByLabelMatch("nApples");
}
Any ideas? Please advice.
Thank you.
Alf P. Steinbach
> Hello,
>
> Please check the below mentioned class. I am looking for a smarter way
> to define the function 'getValueByLabelMatch'. getValueByLabelMatch
> takes a variable label and retrieves the value of the respective class
> member. For simplicity I have used members of int type, but they could
> be of any data type.
>
> /* variable scope deliberately given public scope */
>
> #include <iostream>
> using namespace std;
> class someclass
> {
> public:
> int nApples, nOranges, nGrapes;
Don't make member variables public except in pure data classes (POD structures).
> someclass(int a,int b, int c):nApples(a),nOranges(b),nGrapes(c){}
> int getValueByLabelMatch(string givestr)
Preferentially pass a std::string as 'std::string const& s'.
> {
> if (givestr.compare("nApples")==0 )
Just use '==", that is, the condition 'givestr == "nApples"'.
> return nApples;
> //other if conditions for each of the member variables
> return 0;
> }
> };
> void main()
'main' must always have result type 'int'.
> {
> someclass tst(1,2,3);
> int a = tst.getValueByLabelMatch("nApples");
> }
>
> Any ideas? Please advice.
std:map
Cheers & hth.,
- Alf
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KK
I missed my point a bit. What if the class has 1000 variables instead
of just 3? Is there anyway I could skip the process of creating a map
for each and every variable and instead use an iterator to loop
through all the members until I find a match.
Sam
> I missed my point a bit. What if the class has 1000 variables instead
> of just 3?
Then whoever designed such a class needs to be commited to the looney bin,
and not be allowed anywhere near a C++ compiler.
> Is there anyway I could skip the process of creating a map
> for each and every variable and instead use an iterator to loop
> through all the members until I find a match.
Not in C++. If you want that kind of functionality, use Java. There is no
C++ equivalent of java.lang.reflect.
Chris M. Thomasson
news:96db5677-826b-4bfa...@h11g2000yqb.googlegroups.com...[...]
> I missed my point a bit. What if the class has 1000 variables instead
> of just 3? Is there anyway I could skip the process of creating a map
> for each and every variable and instead use an iterator to loop
> through all the members until I find a match.
Don't you think that using a (hash) map might be faster than iterating
through each and every variable?
James Kanze
> "KK" <pedag...@gmail.com> wrote in message
It's also easier to use. And std::map can be initialized from a
static table, so it's easy to maintain as well (although since
std::map requires dynamic initialization, so there could be an
order of initialization issue).
--
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Pascal J. Bourguignon
> I missed my point a bit. What if the class has 1000 variables instead
> of just 3?
Then definitely use a map.
> Is there anyway I could skip the process of creating a map
> for each and every variable and instead use an iterator to loop
> through all the members until I find a match.
What do you think java does to match the name of the slot to the slot itself?
Yes, a map.
Now if you are complaining that C++ is lower level than Java, there
you get what you asked for.
--
__Pascal Bourguignon__
Thomas Matthews
[snip]
>
> I missed my point a bit. What if the class has 1000 variables instead
> of just 3? Is there anyway I could skip the process of creating a map
> for each and every variable and instead use an iterator to loop
> through all the members until I find a match.
>
Do you actually have a class with 1000 variables?
I've never seen one.
Are you an extremist?
In writing compilers, USB interface parsers and other entities,
I have never had a class with over 1000 variables.
There is another exception and that may be configuration items.
Most of these are of the type <key, value> pairs.
I still haven't put all of them in one class, I group by theme.
--
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KK
wrote:
That was insightful. Is there a sample code which exemplifies this
idea?
No I dont actually use 1000 variables in my code. I just meant to push
the problem to make one think out of the box.
Hendrik Schober
Don't you think that there might be different solutions
depending on whether we're talking 10 members (fine) or
100 members (far off the scale) or 1000 members (simply
ridiculous)?
Schobi
Pascal J. Bourguignon
class WithALotoSlots {
private:
typedef std::map<std::string,Object> SlotMap;
SlotMap slots;
public:
void setSlot(const std::string& slotName,Object& slotValue){
slots[slotname]=slotValue;
}
Object& getSlot(const std::string& slotName){
SlotMap::iterator it=slots.find(slotName);
if(it==slots.end()){
throw std::exception((std::string("Unknown slot named ")+slotName).c_str());
}else{
return(it->second);
}
}
}
void example(){
WithALotoSlots w;
w.setSlot("x")=Object(1.0);
w.getSlot("x").print();
}
--
__Pascal Bourguignon__