What if I don't put a return value to a function whose prototype returns a value [duplicate]

Answer 1

It's undefined behaviour:

[C++11: 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.

It compiles because:

1. the compiler is not required to diagnose it;
2. diagnosing this is not always trivial, so your compiler doesn't bother;
3. C++ is a "do it yourself" language.

Answer 2

If you compile with the flags -Wall -Wextra -pedantic It should give you a compiler warning about a function that should return a value but doesn't do so.