How to transmit a template?

问题

i want a Base template class with 2 template parameters. Specially, second parameter is a template parameter. Derived is derived from Base with CRTP. Now i want to generate the base class of Derived like Base<Derived,Derived::second_tmpl>, but the generating base class isn't same as the real base class of Derived. How do i transmit a template?

#include <type_traits>

template<typename T, template<typename>class U>
struct Base
{
    using type = Base<T,U>;
    using devired_type = T;
    template<typename V>
    using second_tmpl = U<V>;
    using second_type = second_tmpl<type>;
};

template<typename T>
struct Template
{
    using type = Template<T>;
};

struct Derived
    :public Base<Derived,Template>
{
};

//true
static_assert(
        std::is_same<
            Derived::second_type,
            Template<Base<Derived,Template>>>::value,
        "false");
//false
static_assert(
        std::is_base_of<
            Base<Derived,Derived::second_tmpl>,
            Derived
        >::value,
        "false");

template<typename T>
using Template2 = Template<T>;
//false
static_assert(
        std::is_same<
            Base<Derived,Template>,
            Base<Derived,Template2>
        >::value,
        "false");

Use a template which is the same as the original template instead of the original template. The judgement is false;

回答1:

These are the limitations of tempalte template arguments.

Template template arguments are second-class citizens in C++ :(

The second assert should really read

static_assert(std::is_base_of<Base<Derived, Template>, Derived>::value, "false");

which would work.

To combat the problem with the third (the fact that you "can't typedef an open template"), make it a meta-function: e.g. with TemplateGen in the below program:

Live On Coliru

#include <type_traits>

template <typename T, typename UGen>
struct Base {
    using type = Base<T, typename UGen::template type<T> >;
    using devired_type = T;

    template <typename V> using second_tmpl = typename UGen::template type<T> ;
    using second_type = second_tmpl<type>;
};

template <typename T>
struct Template {
    using type = Template<T>;
};

struct TemplateGen {
    template <typename T> using type = Template<T>;
};

struct Derived : public Base<Derived, TemplateGen> {
};

// true
static_assert(std::is_same<Derived::second_type, Template<Derived> >::value, "false");
// false
static_assert(std::is_base_of<Base<Derived, TemplateGen>, Derived>::value, "false");

using Template2 = TemplateGen;

// false
static_assert(std::is_same<Base<Derived, TemplateGen>, Base<Derived, Template2>>::value, "false");

int main(){}

来源：https://stackoverflow.com/questions/27108614/how-to-transmit-a-template

标签

c++

templates

template-templates