使用vs. typedef-有细微的，鲜为人知的区别吗?

本文介绍了使用vs. typedef-有细微的，鲜为人知的区别吗?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

每个人都同意

using <typedef-name> = <type>;

等同于

typedef <type> <typedef-name>;

并且出于各种原因，前者应优先于后者(请参阅Scott Meyers，Effective Modern C ++和有关stackoverflow的各种相关问题).

and that the former is to be preferred to the latter for various reasons (see Scott Meyers, Effective Modern C++ and various related questions on stackoverflow).

这由[dcl.typedef]支持:

This is backed by [dcl.typedef]:

但是，考虑一个声明，例如

However, consider a declaration such as

typedef struct {
    int val;
} A;

在这种情况下，[dcl.typedef]指定:

For this case, [dcl.typedef] specifies:

引用的第3.5节[basic.link]说

The referenced section 3.5 [basic.link] says

假设上面的typedef声明是在全局名称空间中完成的，则结构A将具有外部链接，因为全局名称空间具有外部链接.

Assuming the typedef declaration above is done in the global namespace, the struct A would have external linkage, since the global namespace has external linkage.

现在的问题是，如果根据相同的通用概念将typedef声明替换为别名声明，是否同样成立:

The question is now whether the same is true, if the typedef declaration is replaced by an alias declaration according to the common notion that they are equivalent:

using A = struct {
    int val;
};

尤其是，通过别名声明("using")声明的类型A是否与通过typedef声明声明的类型具有相同的链接?

In particular, does the type A declared via the alias declaration ("using") have the same linkage as the one declared via the typedef declaration?

请注意，[decl.typedef]并不表示别名声明是 typedef声明(它只说这两者都引入了typedef名称)，而[decl.typedef]仅表示具有为链接目的引入 typedef名称的属性的 typedef声明(不是别名声明).如果别名声明不能为链接目的引入typedef名称，则A只是匿名类型的别名，根本没有链接.

Note that [decl.typedef] does not say that an alias declaration is a typedef declaration (it only says that both introduce a typedef-name) and that [decl.typedef] speaks only of a typedef declaration (not an alias declaration) having the property of introducing a typedef name for linkage purposes.If the alias declaration is not capable of introducing a typedef name for linkage purposes, A would just be an alias for an anonymous type and have no linkage at all.

IMO，尽管严格，但这至少是对标准的一种解释.当然，我可能会忽略某些东西.

IMO, that's at least one possible, albeit strict, interpretation of the standard. Of course, I may be overlooking something.

这引起了随后的问题:

如果确实存在这种细微的差异，是出于故意还是出于是对标准的疏忽吗?
编译器/链接器的预期行为是什么?

If there is indeed this subtle difference, is it by intention or isit an oversight in the standard?
What is the expected behavior of compilers/linkers?

下面的由三个文件组成的最小程序(我们至少需要两个单独的编译单元)用于调查问题.

The following minimal program consisting of three files (we need at least two separate compilation units) is used to investigate the issue.

#ifndef A_HPP
#define A_HPP

#include <iosfwd>

#if USING_VS_TYPEDEF
using A = struct {
     int val;
};
#else
typedef struct {
     int val;
} A;
#endif

void print(std::ostream& os, A const& a);

#endif // A_HPP

a.cpp

#include "a.hpp"
#include <iostream>

void print(std::ostream& os, A const& a)
{
   os << a.val << "\n";
}

main.cpp

#include "a.hpp"
#include <iostream>

int main()
{
    A a;
    a.val = 42;
    print(std::cout, a);
}

海湾合作委员会

使用带有"typedef"变体的gcc 7.2对此进行编译，可以干净地编译并提供预期的输出:

GCC

Compiling this with gcc 7.2 with the "typedef" variant compiles cleanly and provides the expected output:

> g++ -Wall -Wextra -pedantic-errors -DUSING_VS_TYPEDEF=0 a.cpp main.cpp
> ./a.out
42

带有"using"变体的编译会产生编译错误:

The compilation with the "using" variant produces a compile error:

> g++ -Wall -Wextra -pedantic-errors -DUSING_VS_TYPEDEF=1 a.cpp main.cpp
a.cpp:4:6: warning: ‘void print(std::ostream&, const A&)’ defined but not used [-Wunused-function]
void print(std::ostream& os, A const& a)
     ^~~~~
In file included from main.cpp:1:0:
a.hpp:16:6: error: ‘void print(std::ostream&, const A&)’, declared using unnamed type, is used but never defined [-fpermissive]
void print(std::ostream& os, A const& a);
     ^~~~~
a.hpp:9:2: note: ‘using A = struct<unnamed>’ does not refer to the unqualified type, so it is not used for linkage
};
 ^
a.hpp:16:6: error: ‘void print(std::ostream&, const A&)’ used but never defined
void print(std::ostream& os, A const& a);
     ^~~~~

看起来GCC遵循上述标准的严格解释，并且在typedef和别名声明之间的链接方面有所不同.

This looks like GCC follows the strict interpretation of the standard above and makes a difference concerning linkage between the typedef and the alias declaration.

使用clang 6，两个变体均可编译并干净运行，而不会发出任何警告:

Using clang 6, both variants compile and run cleanly without any warnings:

> clang++ -Wall -Wextra -pedantic-errors -DUSING_VS_TYPEDEF=0 a.cpp main.cpp
> ./a.out
42

> clang++ -Wall -Wextra -pedantic-errors -DUSING_VS_TYPEDEF=1 a.cpp main.cpp
> ./a.out
42

因此，一个人也可能会问

One could therefore also ask

哪个编译器正确?

鲜为人知的区别吗