编译器错误，还是非标准代码？ - 在lambda中的可变模板捕获 [英] Compiler bug, or non standard code? - Variadic template capture in lambda

问题描述

我有以下C ++ 11代码;

I have the following C++11 code;

template<typename... T>
int g(T... t)
{
    return 0;
}

template<class... Args>
void f(Args... args)
{
    auto lm = [&, args...] { return g(args...); };
    lm();
}

int main()
{
    f(2, 5, 7);
}

我相信它是有效的C ++ 11，根据;

I do believe that it's valid C++11, according to; Section 5.1.2.23 of the standard;

捕获后跟一个省略号是一个
pack扩展（14.5.3）。 [示例：

A capture followed by an ellipsis is a pack expansion (14.5.3). [ Example:

template<class... Args> void f(Args... args) {
    auto lm = [&, args...] { return g(args...); }; lm();
}

- 结束示例]

但是当Clang ++编译得很好时，G ++提供了这个错误;

However while Clang++ compiles fine, G++ provides this error;

main.cpp: In function 'void f(Args ...)':
main.cpp:10:23: error: expected ',' before '...' token
     auto lm = [&, args...] { return g(args...); };
                   ^
main.cpp:10:23: error: expected identifier before '...' token
main.cpp:10:26: error: parameter packs not expanded with '...':
     auto lm = [&, args...] { return g(args...); };
                      ^
main.cpp:10:26: note:         'args'
main.cpp: In lambda function:
main.cpp:10:43: error: expansion pattern 'args' contains no argument packs
     auto lm = [&, args...] { return g(args...); };
                                       ^
main.cpp: In instantiation of 'struct f(Args ...) [with Args = {int, int, int}]::__lambda0':
main.cpp:10:49:   required from 'void f(Args ...) [with Args = {int, int, int}]'
main.cpp:16:14:   required from here
main.cpp:10:19: error: using invalid field 'f(Args ...)::__lambda0::__args'
     auto lm = [&, args...] { return g(args...); };
                   ^

所以我的问题是，这是G ++中的编译器错误吗？ p>

So my question is simply, is this a compiler bug in G++?

推荐答案

看起来支持在GCC中尚未实现。反之亦然，你不能有一个lambda在一个pack扩展（每个包参数产生一个lambda）。

It looks like support has not been implemented in GCC. Vice versa, you cannot have a lambda inside a pack expansion (to produce one lambda per pack argument). It seems the two features don't play together at all.

如果你只是简单地使用 [&] ，则会出现更有帮助的错误消息：

If you simply use [&] instead, then there is a more helpful error message:

抱歉，未实现：在模板中使用'type_pack_expansion'

sorry, unimplemented: use of ‘type_pack_expansion’ in template

免责声明：我的GCC副本是在7月下旬建成的;我可能需要升级。

Disclaimer: My copy of GCC was built in late July; I'm probably due for an upgrade.

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