songyuanyao songyuanyao - 8 days ago 6
C++ Question

Does the C++ standard specify whether a narrowing conversion is an error or warning?

I answered a question, 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

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.

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