由編譯器無法識別的模板函數中的映射迭代器

Question

我有以下代碼。

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給我下面的錯誤。

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」沒有被初始化，Sam Hain在這裏發生了什麼？！

Answer 1

您需要typename關鍵字：

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

參見：Why do we need typename here?

這是因爲你在一個模板定義和iterator是一個從屬名稱。這已經被問過。

g++ "is not a type" error

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

Trouble with dependent types in templates