Partial template binding, create new template as type

Question

Is there some way to partially bind a template to parameter types? For example, I have the following template:

template<typename T, typename Q> struct generic { };

And I have another template which takes a template class as a parameter, expecting to be able to create instances of it with the first type:

template<typename T, template<typename> class Impl>
struct wrapper {
    Impl<T> foo;
};

This would accept a simple template like template<typename T> without changes. What I want to do now is partially bind the generic template, specifying only Q and passing it to wrapper. Making up some syntax, perhaps something like this:

template<typename T> bound = generic<T,some_type>;

I know I can almost get what I want using inheritance:

template<typename T> bound : public generic<T,some_type> { };

I am hoping though to avoid this though as it causes issues with constructors and operators defined in the base class.

`template using bound = generic;` ? AKA "template-typedef" ? — Bartek Banachewicz, Aug 05 '13 at 12:11
@BartekBanachewicz, that looks good... I can't test it though since I'm on GCC 4.6 which doesn't support it yet. That probably means there is no earlier feature which does this. — edA-qa mort-ora-y, Aug 05 '13 at 12:17
That feature is also knowns as *Template Aliases* and according to [this chart](http://gcc.gnu.org/projects/cxx0x.html) it's indeed supported since `4.7`. — Bartek Banachewicz, Aug 05 '13 at 12:21

score 4 · Accepted Answer · edited May 23 '17 at 10:25

4

In C++11 you can use template aliases

template<class X>
using Bind_CPP11 = generic<X, Y>;

template<class X, template<class> class Impl>
struct wrapper_CPP11
{
    Impl<X> foo;
};

In C++98/03, you can use simple class composition (I would not use inheritance here)

template<class X>
struct Bind_CPP03
{
    typedef generic<X, Y> type;
};

template<class X, template<class> class Impl>
struct wrapper_CPP03
{
    typename Impl<X>::type foo;
//  ^^^^^^^^ to extract dependent type
};

Live Example.

edited May 23 '17 at 10:25

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answered Aug 05 '13 at 12:29

TemplateRex

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The composition requires `wrapper` would change how it references the type. I'm not sure if I can do that in my case, but the answer is still fine. – edA-qa mort-ora-y Aug 05 '13 at 12:40
@edA-qamort-ora-y yes, so if you upgrade to C++11, fixing the `wrapper` would be a TODO – TemplateRex Aug 05 '13 at 12:46

Partial template binding, create new template as type

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