std::stringstream is the heavyweight champion. It takes into consideration things like the the stream’s imbued locale, and its functionality involves things like constructing a sentry object for the duration of the formatted operation, in order to deal with exception-related issues. Formatted input and output operations in the C++ libraries have some reputation for being heavyweight, … Read more
Unless otherwise specified, a moved-from object of class type is left in a valid but unspecified state. Not necessarily a “reset state”, and definitely not “invalidated”. For primitive types , moving is the same as copying, i.e. the source is unchanged. The defaulted move-constructor for a class type with primitive members will move each member, … Read more
That’s the simplest and most clear way to write it: template <typename F> auto invoke_log_return(F&& f) { auto result = f(); std::printf(” …logging here… %s\n”, result.foo()); return result; } The GCC gets the right (no needless copies or moves) expected result: s() in main prvalue s() …logging here… Foo! lvalue s(const s&) …logging here… Foo! … Read more
Yes. The trick is to realize that despite the appearance, the portion of a structured binding declaration before the [ doesn’t apply to the names in the identifier-list. They apply instead to the variable introduced implicitly by the declaration. [dcl.struct.bind]/1: First, a variable with a unique name e is introduced. If the assignment-expression in the … Read more
a is not constant expression(see standard quote below) and therefore: a || 1 Is not a constant expression either, although we know the expression has to evaluate to true the standard requires left to right evaluation here and I see no exceptions that would allow the compiler to skip the lvalue-to-rvalue conversion of a. but: … Read more
I think that both are wrong in C++171 and that the expected output should be: 4294967295 0 While the returned value is correct for the latest versions of both compilers, I think that the ios_base::failbit should be set, but I also think there is a confusion about the notion of field to be converted in … Read more
Reflection use cases are outlined in N3814: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3814.html The general view is that we will do it as an extension of the Type Traits library as exemplified by N3815: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3815.html There are two parts to reflection. The first is introspection. Taking an entity and querying constant values about it. The second is reification, which is … Read more
A class with an unsigned char member would not be required by the standard to be the same size or alignment as unsigned char. Whereas the standard requires that enumerations be the same size and alignment as their underlying types. Now, the standard could have simply declared that it is a class type with no … Read more
Firstly, references are not objects. Objects are specified in [intro.object] and references in [dcl.ref]. Subobjects are objects ([intro.object]). Therefore reference members are not subobjects and therefore a class containing only reference members (and no bases) has no subobjects (even though it has data members). [meta.unary.prop] The predicate condition for a template specialization has_unique_object_representations shall be … Read more
How is it useful? It’s useful when you need a copy of *this – for example, when *this itself is no longer valid by the time the lambda is evaluated. How is it different from capturing this? It makes a copy of the object, so that when the lambda is evaluated, its this pointer refers … Read more