Question

Is it possible to write a function that takes a type and returns a (related) type. For instance, a function which take a type called “RandomVariable” and return a type called “RandomVariableCovariance”. I guess in general the question is whether typenames can be parameters or return types. C++0x is fine.

Answer 1

You can’t do it with functions, but you can do it with template specialisations. For example

template <class T>
struct ConvertType;

template <>
struct ConvertType<RandomVariable>
{
    typedef RandomVariableCovariance type;
};

int main()
{
    ConvertType<RandomVariable>::type myVar;
}

Defines a type ConvertType which is specialised to convert from RandomVariable to RandomVariableCovariance. Its possible to do all kinds of clever type selection this way depending on what you need.