Clang：错误：无效使用非静态数据成员 [英] Clang: error: invalid use of non-static data member

问题描述

这个GCC是太好了，做什么Dev认为它会做，或者ang叨对于某事过于lyly。我在标准中缺少一些微妙的规则，其中clang在抱怨这个

时是正确的

或者我应该使用代码的第二位，基本上是offsetof的工作原理

  [adrian @ localhost〜] $ g ++ -Wall -pedantic -ansi a.cc 
 [adrian @ localhost〜] $ a.out 
 50 
 [adrian @ localhost〜] $ cat a.cc 
 #include< iostream> 
 
 struct Foo 
 {
 char name [50]; 
} 
 
 int main（int argc，char * argv []）
 {
 std :: cout< sizeof（Foo :: name）<< std :: endl; 
 return 0; 
} 
 
 
 [adrian @ localhost〜] $ clang ++ a.cc 
 a.cc:10:29：错误：无效使用非静态数据成员'name'
 std :: cout<< sizeof（Foo :: name）<< std :: endl; 
 ~~~~~ ^ ~~~ 
 1错误生成。 
 [adrian @ localhost〜] $ g ++ -Wall -pedantic -ansi b.cc 
 [adrian @ localhost〜] $ a.out 
 50 
 [adrian @ localhost〜] $ cat b.cc 
 #include< iostream> 
 
 struct Foo 
 {
 char name [50]; 
} 
 
 int main（int argc，char * argv []）
 {
 std :: cout< sizeof（static_cast< Foo *>（0） - > name）< std :: endl; 
 return 0; 
} 
 
 
 [adrian @ localhost〜] $ clang ++ b.cc 
 [adrian @ localhost〜] $ a.out 
 50 
  

解决方案

我发现添加 -std = c ++ 11 停止它抱怨。

现代GCC版本允许在 std = c ++ 98 模式。但是，旧版本，如我的GCC 3.3.6， 抱怨并拒绝编译。

现在我不知道C ++ 98的哪个部分违反了这段代码。

维基百科明确指出， C ++ 11，并参考 N2253 < a>，这说明语法最初不被C ++ 98标准视为无效，但随后有意地澄清为不允许这一点（我不知道非静态成员字段与其他变量在数据方面有什么不同）类型）。一段时间后，他们决定使这个语法有效，但是直到C ++ 11。

同一个文档提到了一个丑陋的解决方法，这也可以在整个web：

  sizeof（（Class *）0） - > Field）
  

看起来就像使用 0 ， NULL 或 nullptr 可能会触发编译器警告可能取消引用空指针（尽管事实上 sizeof 它的参数），因此可以使用任意非零值，虽然它看起来像一个反直觉的魔术常量。因此，在我的C ++优雅降级层中，我使用：

  #if __cplusplus> = 201103L 
 #define CXX_MODERN 2011 
 #else 
 #define CXX_LEGACY 1998 
 #endif 
 
 
 #ifdef CXX_MODERN 
 #define CXX_FEATURE_SIZEOF_NONSTATIC 
 #define CxxSizeOf TYPE，FIELD）（sizeof TYPE :: FIELD）
 #else 
 //使用`nullptr`可能会触发警告。 
 #define CxxSizeOf（TYPE，FIELD）（sizeof（reinterpret_cast< const TYPE *>（1234） - > FIELD））
 #endif 
  pre> 
 
 
使用示例：
  //在块级别：
 
 class SomeHeader {
 public：
 
 uint16_t Flags; 
 
 static CxxConstExpr size_t FixedSize = 
 #ifdef CXX_FEATURE_SIZEOF_NONSTATIC 
（sizeof Flags）
 #else 
 sizeof（uint16_t）
 #endif 
; 
 
 
}; // end class SomeHeader 
 
 
 
 //函数内部：
 
 void Foo（void）{
 
 size_t nSize = CxxSizeOf（SomeHeader，Flags）; 
 
} // end function Foo（void）
  
顺便说一句，注意 sizeof（Type）和 sizeof Expression 的语法差异，因为它们在形式上不一样， code> sizeof（Expression）工程 - 只要 sizeof（Expression）有效。因此，最正确和可移植的形式是 sizeof（decltype（Expression）），但不幸的是它只能在C ++ 11中使用;一些遵从者提供 typeof（Expression）很长时间，但这从来不是标准扩展。
 
Is this gcc being overly nice and doing what the dev thinks it will do or is clang being overly fussy about something. Am I missing some subtle rule in the standard where clang is actually correct in complaining about this

Or should I use the second bit of code which is basically the how offsetof works 
[adrian@localhost ~]$ g++ -Wall -pedantic -ansi a.cc
[adrian@localhost ~]$ a.out
50
[adrian@localhost ~]$ cat a.cc
#include <iostream>

struct Foo
{
   char name[50];
};

int main(int argc, char *argv[])
{
   std::cout << sizeof(Foo::name) << std::endl;
   return 0;
}


[adrian@localhost ~]$ clang++ a.cc
a.cc:10:29: error: invalid use of non-static data member 'name'
   std::cout << sizeof(Foo::name) << std::endl;
                       ~~~~~^~~~
1 error generated.
[adrian@localhost ~]$ g++ -Wall -pedantic -ansi b.cc
[adrian@localhost ~]$ a.out
50
[adrian@localhost ~]$ cat b.cc
#include <iostream>

struct Foo
{
   char name[50];
};

int main(int argc, char *argv[])
{
   std::cout << sizeof(static_cast<Foo*>(0)->name) << std::endl;
   return 0;
}


[adrian@localhost ~]$ clang++ b.cc
[adrian@localhost ~]$ a.out
50

解决方案

  
I found adding -std=c++11 stops it complaining. GCC is fine
  with it in either version.
Modern GCC versions allow this even in -std=c++98 mode. However, older versions, like GCC 3.3.6 of mine, do complain and refuse to compile.

  
So now I wonder which part of C++98 I am violating with this code.
Wikipedia explicitly states that such a feature was added in C++11, and refers to N2253, which says that the syntax was not considered invalid by the C++98 standard initially, but then intentionally clarified to disallow this (I have no idea how non-static member fields are any different from other variables with regard to their data type). Some time later they decided to make this syntax valid, but not until C++11.

The very same document mentions an ugly workaround, which can also be seen throughout the web:
sizeof(((Class*) 0)->Field)
It looks like simply using 0, NULL or nullptr may trigger compiler warnings for possible dereference of a null pointer (despite the fact that sizeof never evaluates its argument), so an arbitrary non-zero value might be used instead, although it will look like a counter-intuitive "magic constant". Therefore, in my C++ graceful degradation layer I use:
#if __cplusplus >= 201103L
    #define CXX_MODERN 2011
#else
    #define CXX_LEGACY 1998
#endif


#ifdef CXX_MODERN
    #define CXX_FEATURE_SIZEOF_NONSTATIC
    #define CxxSizeOf(TYPE, FIELD) (sizeof TYPE::FIELD)
#else
    // Use of `nullptr` may trigger warnings.
    #define CxxSizeOf(TYPE, FIELD) (sizeof (reinterpret_cast<const TYPE*>(1234)->FIELD))
#endif
Usage examples:
// On block level:

class SomeHeader {
public:

    uint16_t Flags;

    static CxxConstExpr size_t FixedSize =
#ifdef CXX_FEATURE_SIZEOF_NONSTATIC
        (sizeof Flags)
#else
        sizeof(uint16_t)
#endif
    ;


}; // end class SomeHeader



// Inside a function:

void Foo(void) {

    size_t nSize = CxxSizeOf(SomeHeader, Flags);

} // end function Foo(void)
By the way, note the syntax difference for sizeof(Type) and sizeof Expression, as they are formally not the same, even if sizeof(Expression) works — as long as sizeof (Expression) is valid. So, the most correct and portable form would be sizeof(decltype(Expression)), but unfortunately it was made available only in C++11; some compliers have provided typeof(Expression) for a long time, but this never was a standard extension.

                        
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