Is a return statement mandatory for C++ functions that do not return void?

Question

My Herb Schildt book on C++ says: "... In C++, if a function is declared as returning a value, it must return a value." However, if I write a function with a non-void return type and do not return anything, the compiler issues a warning instead of an error: "Control reaches end of non-void function."

I use gcc (MinGW) and have set the -pedantic flag.

Answer 1

§6.6.3/2:

Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function.

So it depends on your definition of mandatory. Do you have to? No. But if you want your program to have well-defined behavior, yes.*

^{*main is an exception, see §3.6.1/5. If control reaches the end of main without a return, it will have the effect of return 0;.}

Answer 2

It is mandatory--it is an undefined behavior when such function ends without returning anything (so compilers might actually implement some kind of special behaviour). However, there are some special cases.

::main is an exception, it is assumed that return 0; is at the end of its code.

Also, you don't have to return a value in a function that does not return cleanly, f.e.:

int Foo() {
    throw 42;
}