Your program is well-formed and free of undefined behaviour, as far as I can tell. The C++ abstract machine never actually assigns to a const object. A not-taken if() is sufficient to “hide”https://stackoverflow.com/”protect” things that would be UB if they executed. The only thing an if(false) can’t save you from is an ill-formed program, e.g. … Read more
Using a hyperlinked C++ grammar, the parsing of decltype(void()) is: decltype( expression ) decltype( assignment-expression ) decltype( conditional-expression ) … lots of steps involving order of operations go here … decltype( postfix-expression ) decltype( simple-type-specifier ( expression-listopt ) ) decltype( void() ) So void() is a kind of expression here, in particular a postfix-expression. Specifically, … Read more
I posted this in the cpplang slack, and Casey Carter responded: The description of the algorithm in the standard is defective. The intent is to determine [if] every element in the needle appears in order in the haystack. [The algorithm it actually performs is:] “Returns true if the intersection of sorted sequences R1 and R2 … Read more
[variant.visit] in C++17 doesn’t use variant_size_v, but it does in the current working draft as a result of an editorial change. I don’t see any indication that LWG reviewed the change before it went in, but it has looked at this part of the standard several times since then and has yet to object to … Read more
The C90 standard includes getenv(); therefore, the C++98 standard did too. When the C standard was originally created, the precedent for environment setting was putenv(); the setenv() function was not devised until later. The standard committee avoided creating new functions when it could, but also avoided standardizing problematic functions when possible (yes, localeconv() and gets() … Read more
This should actually compile fine for JDK 7 and 8. Quoting JLS section 14.14.2 (which is the same for the Java 7 specification): The enhanced for statement is equivalent to a basic for statement of the form: for (I #i = Expression.iterator(); #i.hasNext(); ) { {VariableModifier} TargetType Identifier = (TargetType) #i.next(); Statement } Rewriting the … Read more
The standard doesn’t say much about the discarded statement of an if constexpr. There are essentially two statements in [stmt.if] about these: In an enclosing template discarded statements are not instantiated. Names referenced from a discarded statement are not required ODR to be defined. Neither of these applies to your use: the compilers are correct … Read more
The C++11 Standard says (ยง10.1) [class.mi]: The order of derivation is not significant except as specified by the semantics of initialization by constructor (12.6.2), cleanup (12.4), and storage layout (9.2, 11.1). The three referenced paragraphs reveal that Constructors are called in the order you write them down (first base class in the list is constructed … Read more
I would have had to vote against your proof-of-concept implementation had it been proposed. I fear it would break the following code, which I’m pretty sure I’ve seen in the wild at least once or twice over the past dozen years. namespace oops { struct foo { foo() : i(0) {} int i; void swap(foo& … Read more
Wrap the existing namespace in a nested namespace which you then “use” in the original namespace. namespace foo { namespace bar_wrapper { enum bar { A }; } using namespace bar_wrapper; } namespace buzz { using namespace foo::bar_wrapper; }