Linking problems in C++

Question

Hi I'm having some trouble with a linking error in C++ and I'm not sure why it is happening, so hopefully someone here can help me, the errors is LNK1120 and LNK2019 unresolved external symbol _main referenced in function "int __cdecl invoke_main(void)" (?invoke_main@@YAHXZ). The code can be seen below:

Cipher.h

#pragma once
#include <string>
#include <algorithm>

namespace ceaser {
    class Cipher
    {
    public:
        std::string FormatMessage(std::string msg);

        std::string Encrypt(std::string msg, int shift);

        std::string Decrypt(std::string encryptedText, int shift);
    };
}

Cipher.cpp

#include "Cipher.h"

namespace ceaser {
    std::string Cipher::FormatMessage(std::string msg) {
        std::transform(msg.begin(), msg.end(), msg.begin(), ::toupper);

        return msg;
    }

    std::string Cipher::Encrypt(std::string msg, int shift) {
        std::string result = "";

        for (int i = 0; i < msg.length(); i++) {
            if (isupper(msg[i]))
                result += char(int(msg[i] + shift - 65) % 26 + 65);
            else
                result += msg[i];
        }

        return result;
    }

    std::string Cipher::Decrypt(std::string encryptedText, int shift) {
        return Cipher::Encrypt(encryptedText, 26 - shift);
    }
}

main.cpp

#include <iostream>
#include "Cipher.h"

namespace ceaser {

    int main()
    {
        Cipher cipher;
        std::string text = "hello world!";
        text = cipher.FormatMessage(text);
        int shift = 4;

        std::string encryptedText = cipher.Encrypt(text, shift);
        std::cout << encryptedText << std::endl;

        std::cout << "decrypting... " + cipher.Decrypt(encryptedText, shift);

        return 0;
    }
}