Re: How to use map containers whith odjects of user classes
On Feb 20, 8:09 am, "Hatzigiannakis Nikos" <ni...@ypai.gr> wrote:
I want to use a map container with objects of the following
class rectangle. Each object will be accosiated with a key of
type string:.
#include <iostream>
#include <string>
#include <map>
using namespace std;
class rectangle
{
float side_a;
float side_b;
public:
float area() {return side_a * side_b;}
rectangle(float a, float b) {side_a=a; side_b=b;}
};
map<string,rectangle> mymap;
main()
{
rectangle rec1(10,20);
//the following statment does not compile
mymap["table"] = rec1;
system("pause");
}
1. The statment mymap["trapezi"] = rec1; supposed to add a new element =
in
the map with the key "table" and value the object rec1 but it doesnt
compile. why?
The sub-expression mymap["trapezi"] returns a reference to the
object. If the object doesn't exist, it creates it, using the
object type's default constructor. Since rectangle doesn't have
a default constructor, it fails. (Note that any use of [] will
fail, even if not being used for insertion.)
The "standard" way to insert an object into a map is with
map<>::insert. The "standard" way to read an object present in
the map is with map<>::find. The [] in map is a convenience
function, for certain specific uses of map, and certain uses
only. (There is no one semantic that it could be given which
would be appropriate for all uses.)
You can either provide rectangle with a default constructor
(probably by specifying default arguments to the present
constructor), or use the "standard" functions insert and find to
access the map. Which solution is best depends on your
application.
2. If I finaly manage to add a new element how can I have
access to the mebers of the rectangle object in a map element?
See above. Either find(), or operator[]. Just be aware that
operator[] will insert an entry if one is not already present,
which may not be what you want.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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