std::list remove function gives compilation error for struct data

Question

#include <iostream>
#include <list>

using namespace std;

struct DATA
{
    int d;
    int d2;
};

int main()
{
    DATA d1;
    d1.d = 100;
    d1.d2 = 200;

    list<DATA> ld;

    ld.push_back(d1);
    ld.remove(d1);
}

Compiling the above code gives following error:

1>------ Build started: Project: ConsoleApplication2_List, Configuration: Debug Win32 ------
1>ConsoleApplication2_List.cpp
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\list(1603,65): error C2676: binary '==': 'const _Ty' does not define this operator or a conversion to a type acceptable to the predefined operator
1>        with
1>        [
1>            _Ty=DATA
1>        ]
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\list(1602): message : while compiling class template member function 'auto std::list<DATA,std::allocator<DATA>>::remove(const _Ty &)'
1>        with
1>        [
1>            _Ty=DATA
1>        ]
1>C:\TestCode\ConsoleApplication2_List\ConsoleApplication2_List\ConsoleApplication2_List.cpp(17): message : see reference to class template instantiation 'std::list<DATA,std::allocator<DATA>>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\list(1614,1): error C2451: conditional expression of type 'void' is illegal
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\list(1614,22): message : Expressions of type void cannot be converted to other types
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\list(1603): message : see reference to function template instantiation 'auto std::list<DATA,std::allocator<DATA>>::remove_if<std::list<DATA,std::allocator<DATA>>::remove::<lambda_c489c5225c742bd5c5073b600e1d767b>>(_Pr1)' being compiled
1>        with
1>        [
1>            _Pr1=std::list<DATA,std::allocator<DATA>>::remove::<lambda_c489c5225c742bd5c5073b600e1d767b>
1>        ]
1>Done building project "ConsoleApplication2_List.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

I have created a C++ program which uses list containing a custom type struct data, it works fine for adding data, but when I call the remove function the compiler issues the above errors.

How do I solve this issue?

Answer 1

As the error message tells you, you need an operator== to check equality. This is how the remove function will know which elements to remove.

struct DATA
{
  int d;
  int d2;

  bool operator==(const DATA& other) const {
      return d == other.d && d2 == other.d2;
  }
};

Edit
As pointed out by Jens it's generally a good idea to define operands as free functions. This is due to the fact that both the right-hand side and left-hand side of the operator would be able to implicitly convert to the correct type. Compared to a member function where only the right-hand side of the operator would be able to implicitly convert.

In this specific case it wouldn't make a difference, but in general it can be a good rule of thumb.

struct DATA
{
  int d;
  int d2;
};

bool operator==(const DATA& lhs, const DATA& rhs) {
  return lhs.d == rhs.d && lhs.d2 == rhs.d2;
}

Answer 2

Based on the following error message, struct DATA needs an == operator method:

C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\list(1603,65): error C2676: binary '==': 'const _Ty' does not define this operator or a conversion to a type acceptable to the predefined operator