C++11：friend 声明扩展

对于friend的用法，C++11新添加了下面两个

friend simple-type-specifier ; //(1)
friend typename-specifier ;    //(2)

对于（1），在C++11之前，下面的代码是无法编译通过的

#include <iostream>

class B
{};

class A
{
  int b;
  friend B;
};

int main()
{
  return 0;
}

你会得到如下的编译错误：

/home/insights/insights.cpp:8:10: warning: unelaborated friend declaration is a C++11 extension; specify 'class' to befriend 'B' [-Wc++11-extensions]
  friend B;
         ^
         class
1 warning generated.

要想编译成功的化，需要将friend B;改为friend class B;。
对于（2），C++11之前同样无法编译通过

#include <iostream>
#include <vector>

class B
{};

typedef std::vector<B> BVec;

class A
{
  int b;
  friend typename BVec::value_type;
};

int main()
{
  return 0;
}

编译错误为：

/home/insights/insights.cpp:12:10: warning: 'typename' occurs outside of a template [-Wc++11-extensions]
  friend typename BVec::value_type;
         ^~~~~~~~~
/home/insights/insights.cpp:12:10: warning: unelaborated friend declaration is a C++11 extension; specify 'class' to befriend 'typename BVec::value_type' (aka 'B') [-Wc++11-extensions]
  friend typename BVec::value_type;
         ^
         class
2 warnings generated.