How to use a single variable which can have multiple data types in C++

In C++ 17 we got something called std::variant it is basicaly a type safe union to use it you just declare the types you need:

std::variant<int, std::string> data;

data = "Hello!";
data = 13;
// No Compilation Issues

The Hard Part Is When You Want To Retrieve The Data

std::string& text = std::get<std::string>(data):
int& number = std::get<int>(data);

BUT MAKE SURE THE LAST THING YOU DEFINED IT AS IS THE TYPE YOU ARE TRYING TO RETRIEVE MEANING:

data = 13;
std::string& text = std::get<std::string>(data); // Bad Exception Error

A better thing is to use std::get_if

if (auto temp = std::get_if<std::string>(&data);
    std::string& text = *temp; // do what ever you want

I Hope this answerd Your Question Happy Coding!

How can a variable have 2 data types?

It cannot. An object has one type. A single type can represent values of different types (unions,std::variant). However, you do not need it here. Whether the user enters 1 or "January" shouldn't matter, because it is both the same month.

What should I do so that the user can enter a string like "January" or enter an integer like 1?

When you read user input, read it as a std::string. Internally you can use an enum:

#include <iostream>
#include <string>

enum class month { January, February, Mazember};

std::istream& operator>>(std::istream& in, month& m){
    std::string temp;
    in >> temp;
    if (temp == "1" || temp == "January") { 
        m = month::January;
    }
    //...
    return in;
}

std::ostream& operator<<(std::ostream& out,const month& m) {
    if (m == month::January) {
        out << "January";
    }
    //...
    return out;
}

int main()
{
    month m;
    std::cin >> m;
    std::cout << m;
}

The declaration will conflict. You can't declare two different data types with a single identifier. For example, std::string month and int month will throw an error. Look at the following:

#include <iostream>

int main(void) {
    int month = 2;
    std::string month = "January"; // confliction
}

You'll get something:

error: conflicting declaration 'std::string month'
    5 |     std::string month = "January";
      |                 ^~~~~
vectors.cpp:4:9: note: previous declaration as 'int month'
    4 |     int month = 2;
      |         ^~~~~

You can overload functions, but not variables.

std::any from c++17 also works.

Here is a little usage example.

//DECLARE ANY VARIABLE.
std::any month;

//GET THE INPUT.
std::string in;
std::cout << "Enter Something : " << std::endl;
std::cin >> in;

//STORE AS AN INTEGER.
if (in == "1")
    month = std::stoi(in);

//STORE AS A STRING
if (in == "January")
    month = in;