SFINAE:当重载移动到其他命名空间时，检查函数是否存在中断 [英] SFINAE: checking the existence of a function breaks when the overload is moved to other namespaces

问题描述

我想使用 SFINAE 检查特定命名空间中是否存在函数.我发现 SFINAE 可以测试来自另一个命名空间的免费函数 可以完成这项工作，但有些事情我不明白.

I want to check for the existence of a function in a specific namespace using SFINAE. I have found SFINAE to test a free function from another namespace which does the job, but there are some things I don't understand.

目前我有这个工作代码，直接来自链接的问题:

Currently I have this working code, straight from the linked question:

// switch to 0 to test the other case
#define ENABLE_FOO_BAR 1

namespace foo {
  #if ENABLE_FOO_BAR
    int bar();
  #endif
}

namespace detail_overload {
  template<typename... Args> void bar(Args&&...);
}
namespace detail {
  using namespace detail_overload;
  using namespace foo;
  template<typename T> decltype(bar()) test(T);
  template<typename> void test(...);
}
static constexpr bool has_foo_bar = std::is_same<decltype(detail::test<int>(0)), int>::value;

static_assert(has_foo_bar == ENABLE_FOO_BAR, "something went wrong");

(ENABLE_FOO_BAR 宏仅用于测试目的，在我的真实代码中我没有这样的宏，否则我不会使用 SFINAE)

(the ENABLE_FOO_BAR macro is just for testing purpose, in my real code I don't have such a macro available otherwise I wouldn't be using SFINAE)

但是，只要我将 detail_overload::bar() 放入任何其他命名空间(根据需要调整 using 指令)，检测就会静默中断，static_assert 在 foo::bar() 存在时 启动.它仅在虚拟" bar() 重载直接位于全局命名空间或 ::detail_overload 命名空间的一部分时才有效(注意全局 :: 范围).

However, as soon as I put detail_overload::bar() in any other namespace (adjusting the using directive as needed), the detection breaks silently and the static_assert kicks in when foo::bar() exists. It only works when the "dummy" bar() overload is directly in the global namespace, or part of the ::detail_overload namespace (note the global :: scope).

// breaks
namespace feature_test {
  namespace detail_overload {
    template<typename... Args> void bar(Args&&...);
  }
  namespace detail {
    using namespace detail_overload;
    using namespace foo;
    //...

// breaks
namespace feature_test {
  template<typename... Args> void bar(Args&&...);
  namespace detail {
    using namespace foo;
    //...

// breaks
namespace detail {
  namespace detail_overload {
    template<typename... Args> void bar(Args&&...);
  }
  using namespace detail_overload;
  using namespace foo;
  //...

// works
template<typename... Args> void bar(Args&&...);
namespace feature_test {
  namespace detail {
    using namespace foo;
    //...

// works
namespace detail_overload {
  template<typename... Args> void bar(Args&&...);
}
namespace feature_test {
  namespace detail {
    using namespace detail_overload;
    using namespace foo;
    //...

我意识到这与我链接到的问题完全相同，并且如前所述，我已经有了一个可行的解决方案，但是没有解决的是 为什么精确地会发生这种情况?

I realize this is the very same problem as the question I linked to, and as mentioned I already have a working solution, but what is not addressed there is why precisely does this happen?

作为一个附带问题，有没有什么方法可以实现正确的 SFINAE 检测而不用 bar() 或 detail_overload 命名空间污染全局命名空间?正如您可以从非工作示例中猜到的那样，我想将所有内容整齐地包装在单个 feature_test 命名空间中.

As a side question, is there any way to achieve correct SFINAE detection without polluting the global namespace with either bar() or a detail_overload namespace? As you can guess from the non-working examples, I'd like to neatly wrap everything in a single feature_test namespace.

推荐答案

我会稍微改变一下，所以 bar 的后备声明不是模板(= 更短的代码)，并且不要使用 SFINAE，因为这纯粹是一个名称查找问题.

I'll change it slightly so the fall-back declaration of bar isn't a template (= shorter code), and don't use SFINAE as this is purely a name lookup issue.

namespace foo {
    int bar(int);
}

namespace feature_test {
    namespace detail_overload {
        void bar(...);
    }

    namespace detail {
        using namespace detail_overload;
        using namespace foo;

        void test() { bar(0); } // (A)
    }
}

在 (A) 行中，编译器需要找到名称 bar.它是如何查找的?它不依赖于参数，所以它必须是非限定查找:[basic.lookup.unqual]/2

In line (A), the compiler needs to find the name bar. How is it looked up? It's not argument-dependent, so it must be unqualified lookup: [basic.lookup.unqual]/2

由 using-directive 指定的命名空间中的声明在包含 using-directive 的命名空间中变得可见；见 7.3.4.出于 3.4.1 中描述的非限定名称查找规则的目的，由 using-directive 指定的命名空间中的声明被视为该封闭命名空间的成员.

The declarations from the namespace nominated by a using-directive become visible in a namespace enclosing the using-directive; see 7.3.4. For the purpose of the unqualified name lookup rules described in 3.4.1, the declarations from the namespace nominated by the using-directive are considered members of that enclosing namespace.

注意它们变成了一个封闭的命名空间，而不是封闭的命名空间.[namespace.udir]/2 的详细信息揭示了这个问题:

Note they become in an enclosing namespace, not the enclosing namespace. The details from [namespace.udir]/2 reveal the issue:

[...] 在非限定名称查找 (3.4.1) 期间，名称看起来好像它们是在最近的封闭命名空间中声明的，该命名空间包含 using-directive 和指定的命名空间.

[...] During unqualified name lookup (3.4.1), the names appear as if they were declared in the nearest enclosing namespace which contains both the using-directive and the nominated namespace.

即对于test里面的bar的名称查找:

That is, for the name lookup of bar inside test:

namespace foo {
    int bar(int);
}

// as if
using foo::bar;
namespace feature_test {
    namespace detail_overload {
        void bar(...);
    }

    // as if
    using detail_overload::bar;
    namespace detail {
        // resolved
        // using namespace detail_overload;
        // using namespace foo;

        void test() { bar(0); } // (A)
    }
}

因此，在 feature_test 中找到的名称 bar 隐藏了在全局范围内找到的名称(不是).

Therefore, the name bar found in feature_test hides the name (not) found in the global scope.

注意:也许您可以通过依赖参数的名称查找(以及第二个 SFINAE)来解决这个问题.如果有什么想到的，我会补充的.

Note: Maybe you can hack around this issue with argument-dependent name lookup (and a second SFINAE). If something comes to my mind, I'll add it.

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