问题描述
考虑以下类定义和：
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
虽然clang ++绝对是错误的（报告为问题）， 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:
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！