Variadic templates typedef

Active Oldest Votes. Improve this answer. Henri Menke Henri Menke I can remember being in my H. I did give you a thumbs : — Francis Cugler. Also think about the embedded developers which are stuck on an old system which isn't upgradable and are forced to use an old c 98 compiler Sign up or log in Sign up using Google.

Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. Note that this is a compile-time template metaprogramming construct - it acts on constants and types, not on runtime objects. Armed with this, we can implement get :. Since it returns a reference, we can use get to both read tuple elements and write to them:. Here is another example I find interesting. It's different from the ones already shown in the article, because it doesn't really use the traditional recursive approach of implementing variadic templates.

Rather, it uses them to express the "any template parameters can go here" concept. Say we want to write a function that can print out standard library containers. We want it to work for any container, and we also want the user to type as little as possible, so we don't want to act on iterators.

Here's a first approach:. Many of the STL containers are templates that can be parameterized by the value type and an allocator type; for example vector , list , deque , and so on. So we can write:. And this works as expected. However, if we try to use it for map , we get a compile error:.

This is because map is a template parameterized by 4 template arguments, not 2. The same problem would occur for a set , which has 3 template arguments. What can we do without duplicating code? Variadic templates for the rescue:.

What this says is - ContainerType is a template template parameter with any amount of template parameters itself. We don't care really, as long as the compiler can type-deduce them at the call. One small addition we have to make to support mappings is:. A somewhat related example is templates that don't do much on their own, but have to forward all their arguments to some other template or function.

Given a generic type T , to invoke the constructor of T , we may need to pass in an arbitrary number of arguments. Unlike function types that specify their arguments at compile time, given just a generic type T we don't know which constructor s it has and how many arguments the constructor accepts. We want to be able to use it as follows:. FooType is an arbitrary type and can be constructed in arbitrary ways. There are two reasons for that. Second, variable argument functions present some difficulties when it comes to implementing type-erasure, which is the technique used in implementing std::function such that it can store whatever is given to it function pointer, lambda, etc.

An incomplete type simply means that the compiler knows that you are invoking a type, but doesn't really know what that type is e. There are certain things you can do with incomplete types, and others that you cannot do with them. Basically, anything that requires any knowledge about the type cannot be done with an incomplete type, such as declaring a variable of that type, calling a member function of it, and things like that.

Some things don't actually require any knowledge about the type and can therefore be done with incomplete types, such as declaring a pointer or reference to that type, making typedefs or type aliases for it, and declaring variables or parameters of that type inside function templates or member functions of class templates.

This is why certain things work and others don't. But in this case, since the specialization of std::function that contains a variable argument function does not exist, there is no way to ever use it in a meaningful way, it doesn't exist!

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Going from "templates" to "variadic templates" is a step unrelated to type aliases. You should learn to break down your problems into smaller chunks! Add a comment. Active Oldest Votes. Improve this answer. SergeyA SergeyA There you go. Again, this answer could do with the basic explanation that using is the new hotness for type aliases, and supports templates By the way, that's two lines ; — Lightness Races in Orbit.

BoundaryImposition, give me 1 min — SergeyA. So obvious, and yet so far from my mind. I don't know what I was thinking. This is the cleanest way. Well, very nearly, anyway. Show 3 more comments.