具有可变参数模板构造函数的推导指南和可变参数类模板-参数包长度不匹配 [英] Deduction guides and variadic class templates with variadic template constructors - mismatched argument pack lengths

问题描述

考虑以下类定义和 推导指南 ：

Consider the following class definition and deduction guide:

template <typename... Ts>
struct foo : Ts...
{
    template <typename... Us>
    foo(Us&&... us) : Ts{us}... { }
};

template <typename... Us>
foo(Us&&... us) -> foo<Us...>;

---

如果我尝试实例化 foo 具有显式的 template参数，代码将正确编译：

---

If I try to instantiate foo with explicit template arguments, the code compiles correctly:

foo<bar> a{bar{}}; // ok

---

如果我尝试实例化 foo 通过扣除指南 ...

foo b{bar{}};

• g ++ 7会产生编译器错误：

  • g++7 produces a compiler error:

    prog.cc: In instantiation of 'foo<Ts>::foo(Us ...) [with Us = {bar}; Ts = {}]':
    prog.cc:15:16:   required from here
    prog.cc:5:27: error: mismatched argument pack lengths while expanding 'Ts'
         foo(Us... us) : Ts{us}... { }
                               ^~~
    

  
  

  • clang ++ 5爆炸：

  • clang++5 explodes:

    #0 0x0000000001944af4 PrintStackTraceSignalHandler(void*) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944af4)
    #1 0x0000000001944dc6 SignalHandler(int) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944dc6)
    #2 0x00007fafb639a390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
    #3 0x0000000003015b30 clang::Decl::setDeclContext(clang::DeclContext*) (/opt/wandbox/clang-head/bin/clang-5.0+0x3015b30)
    ...
    clang-5.0: error: unable to execute command: Segmentation fault
    

  
  

  在线示例wandbox

  虽然clang ++绝对是错误的（报告为问题＃32673 ）， g ++拒绝我的代码正确吗？ 我的代码格式不正确吗？

  While clang++ is definitely bugged (reported as issue #32673), is g++ correct in rejecting my code? Is my code ill-formed?

  推荐答案

  为进一步简化您的示例，看来GCC确实可以不在推导指南中实现可变参数模板参数：

  To simplify your example further, it appears that GCC does not implement variadic template arguments in deduction guides:

  https：/ /wandbox.org/permlink/4YsacnW9wYcoceDH

  我没有在标准或cppreference的演绎指南措辞中明确提及可变参数模板.com。我看不到不允许这样做的标准的解释。因此，我认为这是一个错误。

  I didn't see any explicit mention of variadic templates in the wording for deduction guides in the standard or on cppreference.com. I see no interpretation of the standard that disallows this. Therefore I think this is a bug.

  这篇关于具有可变参数模板构造函数的推导指南和可变参数类模板-参数包长度不匹配的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！