Compile error: Undefined symbols: "_main", referenced from: start in crt1.10.5.o

You have tried to link it already:

g++ file.cpp

That will not only compile it, but try to already create the executable. The linker then is unable to find the main function that it needs. Well, do it like this:

g++ -c file.cpp
g++ -c hasmain.cpp

That will create two files file.o and hasmain.o, both only compiled so far. Now you can link them together with g++:

g++ -omy_program hasmain.o file.o

It will automatically figure out that those are files already compiled, and invoke the linker on them to create a file "my_program" which is your executable.

If you declare the main function in another file, then you must compile the two files separately, and then link them into 1 executable.

Unless you include the entire contents of the file from the file with the main function, that will work too, though a bit odd. But, if you do this then you have to make sure that you compile the file which has the main() function.