问题描述
用旧版本的g++(4.8.0,MinGW)编译项目我发现这段代码编译失败:
Compiling a project with an older version of g++ (4.8.0, MinGW) I found that this code fails to compile:
template<typename T>
void foo() = delete;
template<>
void foo<int>(){}
int main() {
foo<int>();
return 0;
}
如果 g++ 看到基本情况被删除,它似乎甚至不会尝试寻找显式特化.
It seems that g++ doesn't even try to look for explicit specializations if it sees that the base case is deleted.
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.
相反,g++ 4.8.4 和 5.2(在 Linux 上)不会抱怨.这是老版本编译器的BUG还是标准的灰色地带?
Instead, g++ 4.8.4 and 5.2 (on Linux) do not complain. Is this a bug in the older version of the compiler or a gray area in the standard?
附录
clang 3.4.1 似乎也不喜欢它:
clang 3.4.1 too seems not to like it:
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
(和 @Baum mit Augen 在评论中报告说它在 3.7 中仍然不起作用)
(and @Baum mit Augen in the comments reports that it still doesn't work in 3.7)
推荐答案
我不知道以下内容是否有启发,但我发现了 缺陷报告 941:已删除函数模板的显式特化 ,状态为 C++11,声明如下<强>(强调我的):
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):
根据 14.7.3 [temp.expl.spec] 第 1 段,仅未删除函数模板可以被明确地专门化.没有然而,似乎是对这一限制的迫切需要,而且它可能有助于禁止使用隐式实例化专业化,同时仍然允许使用显式专业化版本.
According to 14.7.3 [temp.expl.spec] paragraph 1, only non-deleted function templates may be explicitly specialized. 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.
提议的决议(2010 年 2 月):
Proposed resolution (February, 2010):
将 14.7.3 [temp.expl.spec] 第 1 段更改如下:
Change 14.7.3 [temp.expl.spec] paragraph 1 as follows:
以下任何一项的明确特化:
An explicit specialization of any of the following:
未删除函数模板
类模板
未删除类模板的成员函数
类模板的静态数据成员
类模板的成员类
类或类模板的成员类模板
member class template of a class or class template
未删除 类或类的成员函数模板模板
non-deleted member function template of a class or class
template
可以声明...
现在标准草案的现状N4527 是 14.7.3 显式特化 [temp.expl.spec]:
Now the current state of the draft standard N4527 is 14.7.3 Explicit specialization [temp.expl.spec]:
1 以下任何一项的明确特化:
1 An explicit specialization of any of the following:
(1.1) — 函数模板
(1.1) — function template
(1.2) — 类模板
(1.2) — class template
(1.3) — 变量模板
(1.3) — variable template
(1.4) — 类模板的成员函数
(1.4) — member function of a class template
(1.5) — 类模板的静态数据成员
(1.5) — static data member of a class template
(1.6) — 类模板的成员类
(1.6) — member class of a class template
(1.7) — 类模板的成员枚举
(1.7) — member enumeration of a class template
(1.8) — 类或类模板的成员类模板
(1.8) — member class template of a class or class template
(1.9) — 类或类模板的成员函数模板
(1.9) — member function template of a class or class template
...
所以我猜:
template<typename T>
void foo() = delete;
template<>
void foo<int>(){}
int main() {
foo<int>();
return 0;
}
是 C++11 标准兼容代码,应该被接受.
Is C++11 standard compatible code and should be accepted.
这篇关于删除“通用"的专用模板函数案例无法使用 g++ <=4.8.0 和 clang++ 编译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跟版网!