如何在编译时查询类的所有基类？ [英] How to query for all base classes of a class at compile time?

问题描述

使用 std :: is_base_of< A，B> :: value ，可以检查类 A 基类
class B 。 也可以查询编译器的类
B 的所有基类，例如， base_classes_of< B> 返回包含 B 的所有基类的std :: tuple？

With std::is_base_of<A,B>::value one can check if a class A is a base class of class B. Is it also possible to query the compiler for all base classes of a class B, e.g., something like base_classes_of<B> returning a std::tuple containing all base classes of B ?

有evtl。 g ++中的非标准扩展可以实现这一点？

Is there evtl. a non-standard extension in g++ that can accomplish this ?

如果这是不可能的，有没有人知道为什么？这听起来像一个相当根本的
的信息，编译器很容易应该可用。

If this is not possible at all, does anyone know why? It sounds like a rather fundamental piece of information the compiler easily should have available?

示例：

#include <type_traits>
#include <tuple>

struct A {};
struct B : A {};

static_assert(std::is_base_of<A, B>::value, "A is base of B");
static_assert(! std::is_base_of<B, A>::value, "but B is not base of A");

// now I am looking for something like
// typedef base_classes_of<B>::type B_bases;
// static_assert(std::is_same<B_bases, std::tuple<A>>::value, "all bases of B are: A");

int main() {}

推荐答案

在
基础和 direct_bases www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2965.htmlrel =nofollow> N2965 。

对于数据成员，由于数据成员可以是位字段，它们的类型特征
有一些细微之处。

另一方面，基类没有这样的问题。

我认为在某些情况下对某些编译时查询的需求如 bases
，如N2965的激励示例中所述。

然而，不幸的是，目前的C ++只是缺乏它，就我所见，
GCC和Clang-C ++似乎暂时不提供类似的设施...

Similar facility bases and direct_bases were proposed in N2965.
As for data members, since data members can be bit-fields, their type traits have some subtleties.
On the other hand, base classes don't have such problems.
I think there is the demand on some compile-time query like bases in some situations, as mentioned in Motivating examples of N2965.
However, unfortunately, current C++ just lacks it, and as far as I saw, GCC and Clang-C++ seem not to provide similar facilities at the moment...

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