songyuanyao songyuanyao - 1 year ago 46
C++ Question

Does the standard specify whether compiling error or warning should be generated?

I answered a question here, in which I said for a narrowing conversion an error should be triggered. Because the standard says:

[ Note: As indicated above, such conversions are not allowed at the top level in list-initializations. — end note ]

I think the description of "not allowed" means the compiling should fail.

But another user disagreed, because from here just says "the program is ill-formed", and the standard won't require that compilation must fail.

if a narrowing conversion (see below) is required to convert the
element to T, the program is ill-formed.

So my question is: Does the standard specify whether an error or warning should be generated? Or for some cases the compiling should fail? From the aspect of a compiler, is it OK to make the program compile and just give some warnings?

BTW: Clang 4.0.0 and Gcc 7.0.0 behave differently.

float a {1.e39}; // error for both Clang and Gcc
double d;
float a3{d}; // error for Clang, warning for Gcc

Answer Source

The standard doesn't use the terms "error" and "warning", it only talks about cases where the compiler must "issue a diagnostic".

In your example, if the program is "ill-formed", the compiler is required to tell you that somehow - issue a diagnostic.

After that, it can do anything it likes - including compiling and running the program anyway. The standard only specifies what happens for conforming code, everything else is undefined. And then, as we know, anything can happen.