Using STL maps with custom constructor in a class

[Alexander1234]

Hi,

I've searched many groups and discussions, but nothing I found suited my needs. My problem is rather simple: I want to use a STL map in a class with a custom constructor but don't know how to tell it to use my constructor instead of the default.

The following code illustrates my problem:

#include <stdio.h>
#include <map>
 
class Test {
    private:
        class cmp {
            private:
            public:
                long Ncomp;
                cmp()       {Ncomp = 0;};
                cmp(long n) {Ncomp = n;};
                bool operator() (const char* s1, const char* s2) {
                    return memcmp( s1, s2, Ncomp ) < 0;
                };
        };
        std::map<char*,int,cmp> sols; // like this it uses cmp(),
        // but how to make it use e.g. cmp(5) instead?
    public:
        void AddString(char*);
        void ListStrings(void);
};
 
void Test::AddString(char*s) {
    char*c;
    c = new char[10]; // don't be bothered by the fixed length, it's just an example :)
    memcpy(c,s,10);
    sols[c]++;
}
 
void Test::ListStrings(void) {
    std::map<char*,int,cmp>::iterator it;
    for (it = sols.begin(); it != sols.end(); it++) {
        printf("%s %d\n",it->first,it->second);
    }
}
 
int main() {
    Test t;
    t.AddString("123");
    t.AddString("123");
    t.AddString("234");
    t.ListStrings();
    return 0;
}
 

Outside the class declaration (i.e. in a function declaration) one could use something like

std::map<char*,int,cmp> sols2((5));

but this doesn't work in the declaration, of course.

Hope someone knows better than me,
regards,
Alexander

[Prasannaa]

Hi,
There is no way to associate a particular ctor to a container.
if you take a look at std::map help, the third arg is a comparison function
which helps the container in sorting, it has nothing to with object creation.

One more thing, When you use the map's[] operator , a default object is created(if not already present) and assigned to the key that you used as index, the assignment that you do for this key is calling a copy ctor.

But map also has insert method which takes pair<Key, obj> , you can also use this method to populate the conatiner.Here since you are going to create the object at first place you can use the ctor that u want. Insert won't always insert if the key is already present.Please look at the map's doc for deatils .

Regards
Prasannaa.