检查不完全类型

看muduo库时候，对这里挺有疑问的。

检查不完全类型：

举例如下

//BB.h

 #include
   2 class BB
   3 {
   4 public:
   5     BB()
   6     {
   7         std::cout<<"BB"<
   8     }
   9     ~BB()
  10     {
  11         std::cout<<"~BB"<
  12     }
  13 };

//AA.cpp

  1 #include
  1 #include
  2 class BB;
  3 class AA
  4 {
  5     public:
  6         void deleteBB(BB* pb)
  7         {
  8             delete pb;
  9         }
 10
 11 };
 12
 13
 14 int main()
 15 {
 16     AA a;
 17     //a.deleteBB(new BB);
 18     return 0;
 19 }

当我们在AA.cpp中只声明了class BB，并没有对其定义，这时如果不调用deleteBB（）编译器是不对其报错的，只是warning

AA.cpp: In member function ‘void AA::deleteBB(BB*)’:
AA.cpp:8:11: warning: possible problem detected in invocation of delete operator: [-Wdelete-incomplete]
    delete pb;
           ^
AA.cpp:6:21: warning: ‘pb’ has incomplete type
   void deleteBB(BB* pb)
                     ^
AA.cpp:2:7: note: forward declaration of ‘class BB’
 class BB;
       ^
AA.cpp:8:11: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined
    delete pb;

我们应该尽量让错误在编译时出现，而不是在执行时，所用可以使用一下方法

 void deleteBB(BB* pb)
   7         {
   8             typedef char type_must_be_complete[sizeof(BB) ? 0:-1];
   9             (void) sizeof(type_must_be_complete);
  10             delete pb;
  11         }

如果BB是不完全类型那么sizeof（BB）就应该是0，type_must_be_complete[-1],数组是不能为负数的，所以就会报错

也可以使用boost的checked_array_delete(T * x)；在头文件checked_delete.hpp中；

源码：

 template<class T> inline void checked_array_delete(T * x)
 {
     typedef char type_must_be_complete[ sizeof(T)? 1: -1 ];
     (void) sizeof(type_must_be_complete);
     delete [] x;
 }