How to remove an element from list having user defined data type?

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Hi all,
I am trying to remove an element from C++ list created for user defined data type. It is throwing error since == operator on a user defined type is unknown to template. How do I overcome this problem?

C++

#include <iostream>
#include <list>

using namespace std;

 struct mem_list
    {
        unsigned int addr;
        int size;
    };


int main()
{

    list<mem_list> Mem_numbers;
    mem_list mem;
    mem_list *ptr_mem;
    mem.addr = 0xFF;
    mem.size = 10;

    Mem_numbers.push_front(mem);

    mem.addr = 0xAA;
    mem.size = 20;

    Mem_numbers.push_front(mem);


    mem.addr = 0xBB;
    mem.size = 30;

    Mem_numbers.push_front(mem);


    mem.addr = 0xCC;
    mem.size = 40;

    Mem_numbers.push_front(mem);


    mem.addr = 0xDD;
    mem.size = 50;

    Mem_numbers.push_front(mem);

    int counter = 0;

    for(list<mem_list>::const_iterator it = Mem_numbers.begin(); it != Mem_numbers.end(); ++it)
    {
        mem = *it;
        cout << mem.size<< " ";
    }

    cout << endl;

    for(list<mem_list>::const_iterator it = Mem_numbers.begin(); it != Mem_numbers.end(); ++it)
    {

            mem = *it;

            if( mem.size == 30)
            {
                Mem_numbers.remove(mem);
            }
    }


    for(list<mem_list>::const_iterator it = Mem_numbers.begin(); it != Mem_numbers.end(); ++it)
    {

            mem = *it;
            cout << mem.size<< " ";

    }

    cout << endl;

    return 0;
}

Posted 23-May-11 4:34am

gones

Updated 23-May-11 5:00am

Sandeep Mewara

v2

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Stefan_Lang · Answer 1 · 2011-05-23T05:21:00

The function

std::list<t>::remove(const T&)</t>

searches the list for an element that is equal to the value being passed. To test equality, it invokes the operator

bool operator==(const T& v1, const T& v2)

or, alternately, the operator

bool T::operator==(const T& v) const

For built-in types such as int, the first type of operator is implicitely defined. For user-defined types however it is not. Therefore you must define one of the two forms of this operator yourself:

bool operator==(const mem_list& v1, const mem_list& v2) {
   return v1.addr==v2.addr && v1.size==v2.size;
}
// or:
struct mem_list {
   unsigned int addr;
   int size;
   bool operator==(const struct mem_size& v) const {
      return v.size==size && v.addr==addr;
   }
};(

Alain Rist · Answer 2 · 2011-05-23T05:07:00

Solution 1

Hi,
std::list elements must be equality comparable. Add an operator == () to your struct mem_list like:

C++

struct mem_list
{
    bool operator == (const mem_list& other)
    {
        return (addr == other.addr) && (size == other.size);
    }
    unsigned int addr;
    int size;
};

cheers,
AR
Edited for (hopefully) correct answer

Posted 23-May-11 5:07am

Alain Rist

Updated 23-May-11 5:31am

v2

Comments

Stefan_Lang 23-May-11 11:13am

Actually I believe the compiler automatically adds both copy constructor and assignment operator if you don't do so yourself. (this may be subject to compiler settings though). If it weren't so, you'd get compiler errors for the above code.

Alain Rist 23-May-11 11:33am

You are totally right, the problem is on comparison, see my edited answer. cheers,

How to remove an element from list having user defined data type?

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