I was working on an example for this answer but I forgot to type return *this; to conclude my assignment operator, so Val's assignment operator looks like this:
Val& operator= (const int _a) {a = _a; b = _a + fmod(b, 1.0F);}
It seems like it shouldn't, but gcc compiles and runs this code. How is this possible? Is this like the main function where there's an implied return?
No, there is no implied return, but in default mode g++ does not report missing return as an error. The reason for this is that missing return makes a program ill-formed, and Standard does not require any reporting for ill-formed programs.
You need to make sure you always have following arguments to your g++: -Wall -Wextra -Werror -pedantic. This will make sure gcc will report errors like that and much more.