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c++type-conversionlanguage-lawyer

Does the C++ standard specify that for some cases the compiling should fail with an error?

I'm checking the standard about narrowing conversion, and I think 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 someone told me that 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
Solution

  • 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.