Is there a default return value of C++ functions? [duplicate]

问题

This question already has answers here:

Why does flowing off the end of a non-void function without returning a value not produce a compiler error? (9 answers)

Closed 6 years ago.

I am surprised that a file containing the following lines of code is successfully compiled and linker produces an executable. I thought that all functions, except main, must have a valid return statement unless the return type is void.

int foo(){}
double bar(){}
int main(){}

What am I missing?

回答1:

I thought that all functions, except main must have a valid return statement unless the return type is void.

Yes, they must. You'll get undefined behaviour if they don't.

What am I missing?

If the function is sufficiently complicated, it can be difficult or impossible for the compiler to tell whether all return paths return a value; so the compiler isn't required to diagnose the error.

Most compilers will issue warnings in many cases, if you enable warnings.

回答2:

Not returning a value from a function that says it does is undefined behavior. It compiles and links, but don't expect the program to behave correctly.

If you compile with a high warning level, the compiler will tell you about it.

来源：https://stackoverflow.com/questions/22234357/is-there-a-default-return-value-of-c-functions