How to check if gcc has failed, returned a warning, or succeeded in Bash?

How would I go about checking whether gcc has succeeded in compiling a program, failed, or succeeded but with a warning?

#!/bin/sh

string=$(gcc helloworld.c -o helloworld)

if [ string -n ]; then
    echo "Failure"
else
    echo "Success!"
fi

This only checks whether it has succeeded or (failed or compiled with warnings).

-n means "is not null".

Thanks!

EDIT If it's not clear, this isn't working.