Trying to compile multiple files

[ninja01]

Consider those files:

main.cpp

# include "file.h"

# include <iostream>
using namespace std;

int main()
{
    func();

    cin.get();

    return 0;
}

file.h

# pragma once

# include <iostream>

void func(void);

file.cpp

# include "file.h"

void func(void)
{
    std::cout << "A string" << std::endl;
}

I am trying to build an executable from them. This command(g++ main.cpp -o app.exe) I use gives me the following error:
undefined reference to 'func()'

[salem c]

Try
g++ main.cpp file.cpp -o app.exe

[ninja01]

Thank you @salem