You cannot use auto to store parser expressions¹ Either you need to evaluate from the temporary expression directly, or you need to assign to a rule/grammar: const qi::rule<std::string::const_iterator, qi::space_type> doubles_parser_local = qi::double_ >> *(‘,’ >> qi::double_); You can have your cake and eat it too on most recent BOost versions (possibly the dev branch) there … Read more
The rule for prohibiting non-static members is in 7.1.6.4 clause 4: The auto type-specifier can also be used in declaring a variable in the condition of a selection statement (6.4) or an iteration statement (6.5), in the type-specifier-seq in the new-type-id or type-id of a new-expression (5.3.4), in a for-range-declaration, and in declaring a static … Read more
The type deduction for auto works exactly the same as for templates: when you deduce auto you will get a value type. when you deduce auto& you wil get a non-const reference type when you deduce const auto& you will get a const reference when you deduce auto&& you will get a non-const reference if … Read more
auto can aid performance by avoiding silent implicit conversions. An example I find compelling is the following. std::map<Key, Val> m; // … for (std::pair<Key, Val> const& item : m) { // do stuff } See the bug? Here we are, thinking we’re elegantly taking every item in the map by const reference and using the … Read more
There’s surprisingly little in the standard about this. About all we hear about redeclaration is: [C++11: 3.1/1]: A declaration (Clause 7) may introduce one or more names into a translation unit or redeclare names introduced by previous declarations. [..] and the only relevant part of auto‘s semantics: [C++11: 7.1.6.4/3]: Otherwise, the type of the variable … Read more
Read this article: Appearing and Disappearing consts in C++ Type deduction for auto variables in C++0x is essentially the same as for template parameters. (As far as I know, the only difference between the two is that the type of auto variables may be deduced from initializer lists, while the types of template parameters may … Read more
auto was a keyword that C++ “inherited” from C that had been there nearly forever, but virtually never used because there were only two possible conditions: either it wasn’t allowed, or else it was assumed by default. The use of auto to mean a deduced type was new with C++11. At the same time, auto … Read more
auto and auto && cover most of the cases: Use auto when you need a local copy. This will never produce a reference. The copy (or move) constructor must exist, but it might not get called, due to the copy elision optimization. Use auto && when you don’t care if the object is local or … Read more
Yes, this is an extension. It’s likely to be added to C++17 as part of the ‘concepts’ proposal, I believe.
If you want that to mean that you can pass any type to the function, make it a template: template <typename T> int function(T data); There’s a proposal for C++17 to allow the syntax you used (as C++14 already does for generic lambdas), but it’s not standard yet. Edit: C++ 2020 now supports auto function … Read more