typedefs for templated classes?

Question

Is it possible to typedef long types that use templates? For example:

template <typename myfloat_t>
class LongClassName
{
    // ...
};

template <typename myfloat_t>
typedef std::vector< boost::shared_ptr< LongClassName<myfloat_t> > > LongCollection;

LongCollection<float> m_foo;

This doesn't work, but is there a way to achieve a similar effect? I just want to avoid having to type and read a type definition that covers almost the full width of my editor window.

Answer 1

No, that isn't possible currently. It will be made possible in C++0X AFAIK.

The best I can think of is

template<typename T> struct LongCollection {
    typedef std::vector< boost::shared_ptr< LongClassName<T> > > type;
};

LongCollection<float>::type m_foo;

Answer 2

If you don't want to go the macro way you have to make individual typedefs for each type:

typedef std::vector< boost::shared_ptr< LongClassName<float> > > FloatCollection;
typedef std::vector< boost::shared_ptr< LongClassName<double> > > DoubleCollection;

Answer 3

No, but you can get close using a 'helper' type, see this example.

Answer 4

Its not exactly what you're asking for, but this might achieve the desired effect depending on your actual situation:

template <typename myfloat_t>
class LongClassName
{
    // ...
};

template <typename myfloat_t>
class LongCollection : public std::vector< boost::shared_ptr< LongClassName<myfloat_t> > > 
{
};

You may need to add some constructors or operators depending on your needs.

Answer 5

The solution shown by Leon is canonical. Bit of background knowledge: This is called a “(template) metafunction” because it is basically a “function” that gets evaluated at compile time. Instead of values, it deals with types: There's a list of input types (the type arguments) and there's a “return value”: The typedef that declares the type name “type”.

“Invocation” works analogously to normal function invocation, albeit with a different syntax:

// Normal function
result = f(args);

// Metafunction
typedef f<args>::type result;

This code construct is an often-used idiom in libraries such as the Boost libraries and even in the STL at one place: allocator_type::rebind<U>::other accomplishes the same thing with the only difference that the typedef type is called other.