[SOLVED] c++ how to pass empty list to function parameter?

errigour · 09-27-2022, 10:19 PM

I have this code here but it is complaining about line 38 the Null paramater I am passing to the list argument. How can i pass NULL or at least define it empty?

Code:

#include <iostream>
#include <list>

using namespace std;

template<class T>
void Swap(T&a,T&b){
    T temp = a;
    a=b;
    b=temp;
}

class ilf /* inteliigent life form */ {
    string fn;      // First Name
    string mn;      // Middle Name
    string ln;      // Last Name
    long long int a;     // Age
    list<ilf> sib;  // Siblings
public:
    void dlf (string fname, string mname, string lname, long long int age, list<ilf>sibling) {
        fn = fname;
        mn = mname;
        ln = lname;
        a = age;
        sib=sibling;
    }
};

void print_ft() /* Print Family Tree */ {
}

int main(int argc, char *argv[])
{
    char a='a',b='b';

    ilf am;

    am.dlf("Mwana", "", "", 12500, NULL);

    cout<<a<<" - "<<b<<"\n";
    Swap(a,b);
    cout<<a<<" - "<<b<<"\n";
}

dugan · 09-27-2022, 11:10 PM

NULL is a pointer value. You're passing it by value, so it's not a pointer; it's a list.

If the idea is to make the parameter optional, then you declare the function like this and then you only pass in fname, mname, lname and age:

Code:

void dlf (string fname, string mname, string lname, long long int age, list<ilf>sibling = list<ilf>())

errigour · 09-27-2022, 11:24 PM

After thinking about it maybe the compiler should tell me I am declaring an infinite loop of objects or something cause that's what i was doing i guess. Ok thanks for the pointer.

dugan · 09-27-2022, 11:31 PM

Yeah, that's exactly what happens when a struct has itself as a value member.

It can have pointers to instances of itself though. That's how linked lists are done.

errigour · 09-28-2022, 12:15 AM

Mined if i ask one more question here? on line 36 i can't iterate through the list like I want there are errors what am I doing wrong there?

Code:

#include <iostream>
#include <list>

using namespace std;

template<class T>
void Swap(T&a,T&b){
    T temp = a;
    a=b;
    b=temp;
}

class ilf /* inteliigent life form */ {
public:
    string fn;          // First Name
    string mn;          // Middle Name
    string ln;          // Last Name
    long long int a;    // Age
    bool s;             // Sex false Male True Female
    list<ilf> *sib;     // Siblings

    void dlf (string fname, string mname, string lname, long long int age, bool sex, list<ilf>*sibling) {
        fn = fname;
        mn = mname;
        ln = lname;
        a = age;
        sib=sibling;
    }
};

void print_ft(ilf *pilf, string mname) /* Print Family Tree */ {
    if(!pilf)
        return;
    cout<< pilf->fn << ((mname == "NULL") ? "" : ((pilf->s) ? " Daughter of "+mname : " Son of "+mname))<<endl;

    for(auto i : pilf->sib) {
        print_ft(&i, pilf->fn);
    }
}

int main(int argc, char *argv[])
{
    char a='a',b='b';

    ilf Mwana;
    // Sons of Mwana
    ilf Marvin;
    ilf Ron;
    //Daughter of Mwana Jane
    ilf Jane;

    Marvin.dlf("Marvin", "NULL", "NULL", 12499, 0, NULL);
    Ron.dlf("Ron", "NULL", "NULL", 12498, 0, NULL);
    Jane.dlf("Jane", "NULL", "NULL", 12497, 1, NULL);
    Mwana.dlf("Mwana", "NULL", "NULL", 12500, 2, new list<ilf>{Marvin, Ron, Jane});

    cout<<a<<" - "<<b<<"\n";
    Swap(a,b);
    cout<<a<<" - "<<b<<"\n";

    print_ft(&Mwana, "NULL");
}

errigour · 09-28-2022, 12:21 AM

nevermine i just had to make it

Code:

    if(pilf->sib)
        for(auto i : *pilf->sib) {
            print_ft(&i, pilf->fn);
        }

NevemTeve · 09-28-2022, 10:38 AM

Right now, your 'siblings' list contains copies of the objects, maybe storing pointers would be better. (Actually, every object should be replaced with pointers and some garbage-collection should be used. Oh wait, that is known as "Java").