C++: " multiple definition of 'mainCRTStartup' " error etc - c++

Question

So I was doing some beginner challenges and wanted to modify my code, this was what i did first.
#include <iostream>
#include <stdlib.h>
#include <time.h>
int random;
int guess;
int num_guess = 1;
int main(){
srand(time(NULL));
random = rand() % 100 + 1;
std::cout << "Try to guess my number between 1 and 100." << std::endl;
std::cin >> guess;
while(guess > random){
std::cout << "Sorry too high but i'll give you another try." << std::endl;
std::cin >> guess;
num_guess += 1;
}
while(guess < random){
std::cout << "Sorry too low but i'll give you another try." << std::endl;
std::cin >> guess;
num_guess += 1;
}
if(guess = random){
std::cout << "WOW! Congratulations you actually got it, you did use " << num_guess << " tries tho." << std::endl;
}
return(0);
}
It is supposed to generate a random number between 1 and 100, and then you guess what number it is. But then I copied this code over to another file under the same project because im doing this in school so I wanted all the different versions of my code for documentary purposes. But when I started writing the new code where the program is supposed to guess a number you give it between 1 and 100.
#include <iostream>
#include <stdlib.h>
#include <time.h>
int number;
int guess = 100;
int num_guess = 1;
int main(){
std::cout << "Please enter any number between 1 and 100" << std::endl;
std::cin >> number;
if(number > 100 && number < 1){
std::cout << "Enter a valid number" << std::endl;
std::cin >> number;
}
srand(time(NULL));
guess = rand() % guess + 1;
return(0);
}
I erased the old code from main.cpp and wrote this instead, but when I tried to run it i got these error messages:
multiple definition of `mainCRTStartup'
multiple definition of `WinMainCRTStartup'
multiple definition of `atexit'
multiple definition of `_onexit'
multiple definition of `__gcc_register_frame'
multiple definition of `__gcc_deregister_frame'
undefined reference to `_Jv_RegisterClasses'|

Answer 1

Guess you didn't exclude your old file from the project. In this case linker meets two main functions and doesn't know what to use. Possible ways to solve it:
exclude unused file from the project;
comment out old version;
use conditional compilation:
#ifdef OLD_VER
// main1
...
#else
// main2
...
#endif
create a new project;
use version control systems.
First 3 methods are not recommended for a long use. The last one is a good point (the best point, I think) but it can require elementary VCSs learning.