C++ typedef with nested templates is not a class, struct, or union type

Question

I'm not sure to understand why the following code is not compiled with g++: ``` t.cpp: In instantiation of ‘Distrib’: t.cpp:28:56: instantiated from ‘Sampler > >’ t.cpp:35:48: instantiated from here t.cpp:16:45: erreur: ‘double’ is not a class, struct, or union type t.cpp:18:43: erreur: ‘double’ is not a class, struct, or union type ``` I was expecting to be able to propagate the `AtomType` type across the nested templates… ``` #include #include template class Solution { public: typedef T AtomType; }; template class Distrib { public: typedef typename SOLT::AtomType AtomType; typedef std::vector Matrix; Matrix matrix; }; template class Sampler { public: typedef typename DT::AtomType AtomType; typedef typename Distrib::Matrix Matrix; Matrix matrix; }; int main() { Sampler< Distrib< Solution > > sampler; } ```

Accepted Answer

In your `Sampler` class, you have: ``` typedef typename Distrib::Matrix Matrix; ``` Here, `AtomType` is `double`, so this is ``` typedef typename Distrib::Matrix Matrix; ``` And then in your `Distrib` class, the line ``` typedef typename SOLT::AtomType AtomType; ``` expands to ``` typedef typename double::AtomType AtomType; ``` Hence the error message. I think you want the line in the `Sampler` class to be: ``` typedef typename DT::Matrix Matrix; ```

C++ typedef with nested templates is not a class, struct, or union type

Solution

Problem