混叠boost :: variant或std :: variant无法编译 [英] Aliasing boost::variant or std::variant not compilable

问题描述

当前，我的库使用boost :: optional和boost :: variant.由于C ++ 17已发布，因此我想添加一个选项，使其可以与boost和std一起使用.

Currently my library uses boost::optional and boost::variant. Since C++17 is out, I would like to add an option, that it works with the boost and std.

所以我测试了完整的代码，其中boost可选和variant以及std可选和variant成功.

So I tested the complete code with boost optional and variant and std optional and variant successful.

所以我添加了一个与此相似的头文件:

So I added a header files that is similar to this:

#ifdef USE_BOOST
#include <boost/optional.hpp>
#elif USE_STD
#include <optional>
#endif

namespace Foo {

#ifdef USE_BOOST
template <typename T>
using optional = boost::optional<T>
#elif USE_STD
template <typename T>
using optional = std::optional<T>
#endif

}

一切正常.

然后我为变体添加了类似的内容

Then I added something similar for variant

#ifdef USE_BOOST
#include <boost/variant.hpp>
#elif USE_STD
#include <variant>
#endif

namespace Foo {

#ifdef USE_BOOST
template <typename... T>
using variant = boost::variant<T...>
// similar to the following
#elif USE_STD

template <typename...T>
using variant = std::variant<T...>;

template <class T, class... Types>
constexpr T& get(std::variant<Types...>& v) {
    return std::get<T>(v);
};

template <class T, class... Types>
constexpr T&& get(std::variant<Types...>&& v) {
    return std::get<T>(std::move(v));
};

template <class T, class... Types>
constexpr const T& get(const std::variant<Types...>& v) {
    return std::get<T>(v);
};

template <class T, class... Types>
constexpr const T&& get(const std::variant<Types...>&& v) {
    return std::get<T>(std::move(v));
};

#endif

}

在库中，我现在到处都使用Foo :: optional和Foo :: variant.但是现在所有功能模板，例如

And within the library I use now everywhere Foo::optional and Foo::variant. But now all function templates like

template <typename... Args>
bool bar(const std::tuple<variant<Args,
                                  std::exception_ptr>...>& args)

找不到

了. (c语错误消息:没有匹配的函数可以调用...)

are not found any more. (clang error message: no matching function for call to ...)

经过clang干线和VS 2017 15.6.1预览版测试 有什么主意吗?

Tested with clang trunk and VS 2017 15.6.1 Preview Any idea?

非常感谢！

我的要点

推荐答案

最后解决了问题.这是在VS中lang的错误. GCC编译就可以了. clang和VS的解决方案是使用显式模板参数调用该函数.

Finally problem solved. It is a bug in clang an VS. GCC compiles it fine. The solution for clang and VS is calling the function with explicit template parameters.

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