const accessor to map?

[nw]

Hi comp.lang.c++,

I'm using map to store a bunch of vectors. I'd like to be able to
return const references to the vectors from my object. However I can't
see a way of doing this with map, as the vectors aren't const. Is
there a way round this, or a better container I should be using?

Advice appreciated.

Example follows:

#include <iostream>
#include <vector>
#include <map>

using namespace std;

class MyMapEncap {
public:

map<string,vector<int> > mymap;

const vector<int> &getvec(string id) const { // Doesn't work because
this isn't allow to be const
// I
want to keep it const is there a work around?
return mymap[id];
}
};

int main() {
MyMapEncap e;

e.getvec("RAW");

}

 

[nw]

I figured it out. Basically use find to access the elements:

#include <iostream>
#include <vector>
#include <map>

using namespace std;

class MyMapEncap {
public:

map<string,vector<int> > mymap;

const vector<int> &getvec(string id) const {
return (*mymap.find(id)).second;
}
};

int main() {
MyMapEncap e;

e.getvec("RAW");

}

 

[Stefan Naewe]

I figured it out. Basically use find to access the elements:

#include <iostream>
#include <vector>
#include <map>

using namespace std;

class MyMapEncap {
public:

map<string,vector<int> > mymap;

const vector<int> &getvec(string id) const {
return (*mymap.find(id)).second;
}
};

int main() {
MyMapEncap e;

e.getvec("RAW");

Crash! Boom! Bang!

}

You need to check the return value of mymap.find() !!

/S

 

[Harsh Puri]

Crash! Boom! Bang!




You need to check the return value of mymap.find() !!

/S
--
Stefan Naewe            stefan dot naewe at atlas-elektronik dot com
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Yes and probably you should think about passing the vector as a
parameter by reference, rather than returning it from the function.

In the latter you need to worry about what to return if the string id
is not in the map.