在clang中使用别名模板时，有没有办法缩短模板化的类名？

Question

这是对上一个问题here 的跟进。基本上就是下面这个程序

#include <memory>

// Create a class parameterized on a template
template <template <typename> class XX>
struct Foo{};

// Some template with a long name
template <typename T>
struct ReallyLongFileNameThatIHateToType {
};

// Alias to shorten name
template <typename T>
using Bar = ReallyLongFileNameThatIHateToType <T>;

int main() {
    std::unique_ptr<Foo<ReallyLongFileNameThatIHateToType>> foo =
        std::make_unique<Foo<Bar>>();
}

在 gcc 上工作得很好，而不是 clang

g++ -std=c++14 test04.cpp -o test04
clang++ -std=c++14 test04.cpp -o test04
test04.cpp:20:61: error: no viable conversion from 'unique_ptr<Foo<template
      Bar>>' to
      'unique_ptr<Foo<template ReallyLongFileNameThatIHateToType>>'
    std::unique_ptr<Foo<ReallyLongFileNameThatIHateToType>> foo =
                                                            ^
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/unique_ptr.h:200:17: note: 
      candidate constructor not viable: no known conversion from 'typename
      _MakeUniq<Foo<Bar> >::__single_object' (aka 'unique_ptr<Foo<Bar> >') to
      'nullptr_t' for 1st argument
      constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
                ^
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/unique_ptr.h:205:7: note: 
      candidate constructor not viable: no known conversion from 'typename
      _MakeUniq<Foo<Bar> >::__single_object' (aka 'unique_ptr<Foo<Bar> >') to
      'std::unique_ptr<Foo<ReallyLongFileNameThatIHateToType>,
      std::default_delete<Foo<ReallyLongFileNameThatIHateToType> > > &&' for 1st
      argument
      unique_ptr(unique_ptr&& __u) noexcept
      ^
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/unique_ptr.h:356:7: note: 
      candidate constructor not viable: no known conversion from 'typename
      _MakeUniq<Foo<Bar> >::__single_object' (aka 'unique_ptr<Foo<Bar> >') to
      'const std::unique_ptr<Foo<ReallyLongFileNameThatIHateToType>,
      std::default_delete<Foo<ReallyLongFileNameThatIHateToType> > > &' for 1st
      argument
      unique_ptr(const unique_ptr&) = delete;
      ^
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/type_traits:1957:41: note: 
      candidate template ignored: disabled by 'enable_if' [with _Up = Foo<Bar>,
      _Ep = std::default_delete<Foo<Bar> >]
    using _Require = typename enable_if<__and_<_Cond...>::value>::type;
                                        ^
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/unique_ptr.h:228:2: note: 
      candidate template ignored: could not match 'auto_ptr' against
      'unique_ptr'
        unique_ptr(auto_ptr<_Up>&& __u) noexcept;
        ^
1 error generated.
Makefile:2: recipe for target 'all' failed
make: *** [all] Error 1

正如链接消息中所指出的，这是 CWG 问题 1244。无论如何，还有其他方法可以缩短与 clang 一起使用的名称吗？通常，我在模板类上对代码进行了参数化，但我不想一遍又一遍地输入完整的参数名称。使用 gcc 时，我只是创建别名模板并收工。显然，对于 clang，这是行不通的，那么是否有不同的机制来实现相同的效果？

Answer 1

作为一种解决方法，您可以通过将模板模板模板参数传递给一些帮助器结构以使用您的模板模板参数进行设置来使其以其他方式工作，例如（在clang++，c++14中工作）：

#include <memory>

// Create a class parameterized on a template
template <template <typename> class XX>
struct Foo{};

// Some template with a long name
template <typename T>
struct ReallyLongFileNameThatIHateToType {
};

template <template <template <class> class> class FF>
struct Bar {
  using type = FF<ReallyLongFileNameThatIHateToType>;
};

template <template <template <class> class> class FF>
using Bar_t = typename Bar<FF>::type;


int main() {
    std::unique_ptr<Foo<ReallyLongFileNameThatIHateToType>> foo =
        std::make_unique<Bar_t<Foo>>();
}