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c++templatesc++11typedeftemplate-aliases

What was the issue solved by the new "using" syntax for template typedefs?

In C++11 you can create a "type alias" by doing something like 

template <typename T>
using stringpair = std::pair<std::string, T>;

But this is a deviation from what you'd expect a template typedef would look like:

template <typename T>
typedef std::pair<std::string, T> stringpair;

So this raises the question - why did they need to come up with a new syntax? what was it that did not work with the old typedef syntax?

I realize the last bit doesn't compile but why can't it be made to compile?

Solution

  • From the WG21 proposal N1489 Template aliases (by Stroustrup and Dos Reis):

    It has been suggested to (re)use the keyword typedef as done in the paper [4] to introduce template aliases:

     template<class T> 
 typedef std::vector<T, MyAllocator<T> > Vec;

    That notation has the advantage of using a keyword already known to introduce a type alias. However, it also displays several disavantages among which the confusion of using a keyword known to introduce an alias for a type-name in a context where the alias does not designate a type, but a template; Vec is not an alias for a type, and should not be taken for a typedef-name. The name Vec is a name for the family std::vector<o, MyAllocator<o> > where the bullet is a placeholder for a type-name. Consequently we do not propose the typedef syntax.

    On the other hand the sentence

    template<class T> 
using Vec = std::vector<T, MyAllocator<T> >;

    can be read/interpreted as: from now on, I'll be using Vec<T> as a synonym for std::vector<T, MyAllocator<T> >. With that reading, the new syntax for aliasing seems reasonably logical.

    The paper [4] referred to in the above quote was a prior proposal WG21 N1406 Proposed Addition to C++: Typedef Templates (by Herb Sutter). It uses both a different syntax (typedef vs using) as well as a different nomenclature (typedef templates vs template aliases). Herb's proposed syntax didn't make it, but the nomenclature can sometimes be found in informal discussions.