相关疑难解决方法(0)

在模板,在那里,为什么我必须把typename和template上依赖的名字呢？究竟什么是依赖名称？我有以下代码:

template <typename T, typename Tail> // Tail will be a UnionNode too.
struct UnionNode : public Tail {
    // ...
    template<typename U> struct inUnion {
        // Q: where to add typename/template here?
        typedef Tail::inUnion<U> dummy; 
    };
    template< > struct inUnion<T> {
    };
};
template <typename T> // For the last node Tn.
struct UnionNode<T, void> {
    // ...
    template<typename U> struct inUnion {
        char fail[ -2 + (sizeof(U)%2) ]; // Cannot be instantiated for any …

我有以下代码.

template<class key,class val>
bool has_key(key chkey,std::map<key,val> map){
  for (std::map<key,val>::iterator it = map.begin(); #line 13 referenced by gcc
      it!=map.end(); ++it){
    if(chkey == it->first) return true;
  }
  return false;
}

海湾合作委员会给我以下错误.

objects.hpp: In function `bool has_key(key, std::map<key, val, std::less<_Key>,
  std::allocator<std::pair<const _Key, _Tp> > >)':
objects.hpp:13: error: expected `;' before "it"
objects.hpp:14: error: `it' was not declared in this scope

某种方式"它"没有被初始化,山姆海恩在这里发生了什么？!