[英]Specialized template function with deleted “general” case fails to compile with g++ <=4.8.0 and clang++
Compiling a project with an older version of g++ (4.8.0, MinGW) I found that this code fails to compile: 使用旧版本的g ++(4.8.0,MinGW)编译项目时,我发现此代码无法编译:
template<typename T>
void foo() = delete;
template<>
void foo<int>(){}
int main() {
foo<int>();
return 0;
}
It seems that g++ doesn't even try to look for explicit specializations if it sees that the base case is deleted. 如果g ++看到基本情况已删除,似乎甚至都不会尝试寻找显式的专业化。
mitalia@mitalia:~/scratch$ /opt/mingw32-dw2/bin/i686-w64-mingw32-g++ -std=c++11 buggy_deleted_template.cpp
buggy_deleted_template.cpp: In function 'int main()':
buggy_deleted_template.cpp:8:14: error: use of deleted function 'void foo() [with T = int]'
foo<int>();
^
buggy_deleted_template.cpp:5:6: error: declared here
void foo<int>(){}
^
mitalia@mitalia:~/scratch$ /opt/mingw32-dw2/bin/i686-w64-mingw32-g++ --version
i686-w64-mingw32-g++ (rubenvb-4.8.0) 4.8.0
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Instead, g++ 4.8.4 and 5.2 (on Linux) do not complain. 相反,g ++ 4.8.4和5.2(在Linux上)不会抱怨。 Is this a bug in the older version of the compiler or a gray area in the standard? 这是旧版编译器中的错误还是标准中的灰色区域?
Addendum 附录
clang 3.4.1 too seems not to like it: clang 3.4.1似乎也不喜欢它:
mitalia@mitalia:~/scratch$ clang++ -std=c++11 buggy_deleted_template.cpp
buggy_deleted_template.cpp:5:6: error: redefinition of 'foo'
void foo<int>(){}
^
buggy_deleted_template.cpp:5:6: note: previous definition is here
buggy_deleted_template.cpp:8:5: error: no matching function for call to 'foo'
foo<int>();
^~~~~~~~
buggy_deleted_template.cpp:2:6: note: candidate template ignored: substitution failure [with T = int]
void foo() = delete;
^
2 errors generated.
mitalia@mitalia:~/scratch$ clang++ --version
Ubuntu clang version 3.4-1ubuntu3 (tags/RELEASE_34/final) (based on LLVM 3.4)
Target: x86_64-pc-linux-gnu
Thread model: posix
(and @Baum mit Augen in the comments reports that it still doesn't work in 3.7) (并且@Baum mit Augen在评论中报告它在3.7中仍然不起作用)
I don't know if the following will be enlightening but I found defect report 941: Explicit specialization of deleted function template with status C++11 that states the following (Emphasis Mine) : 我不知道以下内容是否可以启发您,但我发现了缺陷报告941:状态为C ++ 11 的已删除功能模板的显式专业化,其内容如下: (Emphasis Mine) :
According to 14.7.3 [temp.expl.spec] paragraph 1, only non-deleted function templates may be explicitly specialized. 根据14.7.3 [temp.expl.spec]段落1,只有未删除的功能模板可以显式专用。 There doesn't appear to be a compelling need for this restriction, however, and it could be useful to forbid use of implicitly-instantiated specializations while still allowing use of explicitly-specialized versions. 但是,似乎没有迫切需要此限制,并且在仍然允许使用显式专门化版本的情况下,禁止使用隐式实例化的专门化功能可能很有用。
Proposed resolution (February, 2010): 拟议的决议(2010年2月):
Change 14.7.3 [temp.expl.spec] paragraph 1 as follows: 将14.7.3 [temp.expl.spec]段落1更改如下:
An explicit specialization of any of the following: 下列任何一项的显式专业化:
non-deleted 未删除function template 功能模板class template 类模板
non-deleted 未删除member function of a class template 类模板的成员函数static data member of a class template 类模板的静态数据成员
member class of a class template 类模板的成员类
member class template of a class or class template 类的成员类模板或类模板
non-deleted 未删除member function template of a class or class template 类的成员函数模板或类模板can be declared... 可以声明...
Now the current state of the draft standard N4527 is 14.7.3 Explicit specialization [temp.expl.spec]: 现在,标准草案N4527的当前状态为14.7.3显式专业化[temp.expl.spec]:
1 An explicit specialization of any of the following: 1以下任何一项的显式专业化:
(1.1) — function template (1.1)—功能模板
(1.2) — class template (1.2)-类模板
(1.3) — variable template (1.3)-变量模板
(1.4) — member function of a class template (1.4)-类模板的成员函数
(1.5) — static data member of a class template (1.5)-类模板的静态数据成员
(1.6) — member class of a class template (1.6)-类模板的成员类
(1.7) — member enumeration of a class template (1.7)—类模板的成员枚举
(1.8) — member class template of a class or class template (1.8)-类或类模板的成员类模板
(1.9) — member function template of a class or class template (1.9)—类或类模板的成员函数模板
... ...
So I guess: 所以我猜:
template<typename T>
void foo() = delete;
template<>
void foo<int>(){}
int main() {
foo<int>();
return 0;
}
Is C++11 standard compatible code and should be accepted. 是C ++ 11标准兼容的代码,应被接受。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.