由编译器无法识别的模板函数中的映射迭代器

Question

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; 
} 

GCC给我下面的错误。

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 

不知何故“它”没有被初始化，Sam Hain在这里发生了什么？！

Answer 1

您需要typename关键字：

for (typename std::map<key,val>::iterator it = map.begin(); #line 13 referenced by gcc 
    it!=map.end(); ++it){ 

参见：Why do we need typename here?

这是因为你在一个模板定义和iterator是一个从属名称。这已经被问过。

g++ "is not a type" error

C++ Template: 'is not derived from type'

Trouble with dependent types in templates