Hello,
I was wondering why when compiling the CPP code in Unreal Engine, if a function that needs to return a variable does not return it, the compiler does not compile while in the standard cpp it seams to be optional to return a variable such as int Main() { return 0; } for example.
Thank you!
Hello Philx
With the Main function, the compiler will add a return call if one is not present (defaulting to return 0) as with Main you can only enter and exit that function once and is the entry point for C++ applications.
With functions other than Main, the compiler doesnāt have a default option (how could it? it doesnāt know what your intention is for that function) so it gives an error during compilation.
I hope this helps, good luck on your learning journey!
Thank you very much for taking time to answer!
@Tuomo_T @Philx I am surprised that
int main()
{ }
has an implicit return 0; in there. Is that part of the language definition or just something that a compiler implementation does to get past a standard situation?
I would have thought that the absence of a return with result of the type specified for the function would be at least a warning everywhere.
Now I am curious to see what happens with other cases, such as a void function in which there is a return.
OK, I get it. A return statement can have no result expression. And for a void-returning function, return; is enough, and there is an impllicit value-less return from any function where operation falls through the function body. The idea of an implicit return 0; for an int main() is apparently a compiler feature. Hereās what Microsoft says about it: return Statement (C) | Microsoft Docs at least as recently as VS 2019.
@orcmid I created a new function before the main function to test it with int return and it result with a warning but it compile without any return. Thanks for the link!
I realize Iām late to the party. But for the record adding āreturnā to a void function just exits that function. Which is both valid and may be desirable based on what your function does.
Keep in mind that if you do call return in a void function, trying to return a value will cause a compiler error.
@Tuomo_T understandably. However, int main(void) is not a void function. It has a specified return type. C99 is pretty strict about matching of returns with the specified return type, requiring no expression if the return type is āvoidā. The case at hand is if a function drops through the end of its body without any return. From Harbison & Steele ed.5,
If program control reaches the end of a function body without encountering a
returnstatement, then the effect is as if areturnstatement with no expression were executed. If the function has a non-voidreturn type, then the behavior is undefined.
That appears to be the gap that allows implementation-dependent implementations of main to imply return 0; although one should not rely on it.
Hereās a consideration that bothers me more. It is possible to invoke a non-void function and do so in a statement that does not employ a returned value. This happens all the time in the case of int-returning functions where the invoking code is essentially ignoring any returned result because it is the side-effect of the operation that is all that matters (i.e., printf and such). This is so commonplace, compilers donāt warn about it. Is the same true of other numeric returns, how about non-numeric returns (i.e., returning a struct value)? These are all cases of generally-permissible discarded values. I would expect some sort of warning in some cases, at least as valuable as ones about slack bytes in structs 
PS: I only have information as recent as C99. I donāt know if later revisions of ISO C Language have altered any of this.