So I understand using var in C# makes sense because you have anonymous types that are compiler derived. C++ doesn't seem to have this feature (unless I'm wrong), so what is the point of having an auto keyword?
(It is kinda cool that unlike C#, auto does work for member/global variables, which is cool I guess, but doesn't seem enough to justify its existence).
auto has a lot of uses when it comes down to both generic programming and to save the programmer some typing.
For example, consider this. Would you rather type out:
std::unique_ptr<name::long_type::goes_here> g =
std::make_unique<name::long_type::goes_here>(1,2,3,4)
or:
auto g = std::make_unique<name::long_type::goes_here>(1,2,3,4)
Yes, they're both long but we know the return type and specifying it again is a bit cumbersome to type. This also goes for iterators:
for(auto i = vec.begin(); ...)
vs:
for(std::vector<type>::iterator i = vev.begin(); ...)
Its use in generic programming is also to figure out the return type of a function or if you're doing some generic algorithms where you don't know the type.
For example, consider a very basic example.
template<typename T, typename U>
auto add(T t, U u) -> decltype(t + u) {
return t + u;
}
This allows the compiler to figure out the type of the add operation rather than us trying to figure it out ourselves. Note that in C++14 you can omit the trailing return type. Its uses in generic programming don't stop there either. If we wanted to work with any type of container as a wrapper function for algorithms we could use auto to help us with it. For example:
template<class Cont>
void my_sort(Cont&& cont) {
using std::begin;
auto first = begin(std::forward<Cont>(cont));
// work with the iterators here
}
In the future (C++14), auto can be used to make polymorphic lambdas as well such as:
[](auto a) { return a + 4; }
Which can be useful as well.
