How to write a class template to handle different data members?

I would like to write a class template that behaves as follows:

• If I pass Type_1 as an argument of the class instance, having then something like TestClass<MessageType::Type_1> t1, I want to see both memebers of the struct Message.

• If I pass Type_2, I want to see only the integer member_2 and NOT the member_1 inside the Message struct.

So, in a nutshell, I would like to decide the what are the members according to the template parameter.

#include <string>

enum class MessageType
{
    Type_1,
    Type_2
};

template <MessageType messageType>
class TestClass
{
public:
    struct Message
    {
        std::string member_1;
        int member_2{0};
    };     
};

I've tried to use conditional statements but I got compiler errors

Whether the following is viable in your case depends. If possible I would refactor to have two different types and then pick one via specialization of a separate template:

#include <iostream>
#include <string>

enum class MessageType
{
    Type_1,
    Type_2
};

struct message1 { std::string member1; int member2; };
struct message2 { std::string member1; };

template <MessageType mt> struct select_type;
template <> struct select_type<MessageType::Type_1> { using type = message1; };
template <> struct select_type<MessageType::Type_2> { using type = message2; };


template <MessageType select>
struct TestClass
{
    using message_type = typename select_type<select>::type;
};

int main()
{
    return 0;
}