Oliver Kullmann wrote:
Hello,
Consider the following basic task: Given an integer variable "n"
int n;
and an input stream "in"
std::istream in;
read an integer from in and put it into n. The "solution"
in >> n;
is not a solution, since according to 27.6.1.2.2 from the C++ standard the locale's num_get<> object is invoked, which according to 22.2.2.1.2 from the C++ standard invokes scanf from the C library, Actually, it just says it should behave /like/ scanf: it is defined in terms of it, not (necessarily) implemented with it. which then according to 7.19.6.2 from the C standard, clause 10, yields undefined behaviour if the number represented in the stream is not representable by the type int. The standard specifies that when input fails (like a formatted input operation not finding the right format), the stream's failbit is set, so just: in >> n; if(in.fail()) {...} does what you want. (The more general "if(!in)" works as well, also including a corrupted stream and EOF)
Thus, since "in" represents here user input, and user input shall never yield undefined behaviour, we cannot use "in >> n;". I assume you mean "user input shall never yield *defined* behaviour" ;-)