Return type forwarding in generic code For non-generic code, like the initial example you gave, you can manually select to get a reference as a return type: auto const& Example(int const& i) { return i; } but in generic code you want to be able to perfectly forward a return type without knowing whether you … 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
In C++11, there are two syntaxes for function declaration: return-type identifier ( argument-declarations… ) and auto identifier ( argument-declarations… ) -> return_type They are equivalent. Now when they are equivalent, why do you ever want to use the latter? Well, C++11 introduced this cool decltype thing that lets you describe type of an expression. So … Read more
By using auto&& var = <initializer> you are saying: I will accept any initializer regardless of whether it is an lvalue or rvalue expression and I will preserve its constness. This is typically used for forwarding (usually with T&&). The reason this works is because a “universal reference”, auto&& or T&&, will bind to anything. … 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
I think that one should use the auto keyword whenever it’s hard to say how to write the type at first sight, but the type of the right hand side of an expression is obvious. For example, using: my_multi_type::nth_index<2>::type::key_type::composite_key_type:: key_extractor_tuple::tail_type::head_type::result_type to get the composite key type in boost::multi_index, even though you know that it is … Read more