C ++ const std :: map引用编译失败 [英] C++ const std::map reference fails to compile

问题描述

有一个原因，为什么传递引用STL映射作为const导致[]运算符破裂？我使用const时得到这个编译器错误（gcc 4.2）：

错误：
'map [name]'

这是函数原型：

  void func（const char ch，std :: string& str，const std :: map< std :: string，std :: string>& map）; 
  

而且，我应该提到当我删除 const 。

如果我被正确指示，[]操作符将实际插入一个新对如果它没有找到钥匙，这当然会解释为什么会发生，但我不能想象这将是可以接受的行为。

如果有一个更好的方法，如使用 find 而不是[]，我会很感激。我似乎无法找到工作，不过...我收到 const 不匹配的迭代器错误。

谢谢。

解决方案

是的，您不能使用 operator [] 。使用 find ，但注意返回 const_iterator 而不是 iterator ：

  std :: map< std :: string，std :: string> :: const_iterator it; 
 it = map.find（name）; 
 if（it！= map.end（））{
 std :: string const& data = it-> second; 
 // ... 
} 
  

就像指针。您不能将 int const * 分配给 int * 。同样，您不能将 const_iterator 分配给迭代器。

Is there a reason why passing a reference to a STL map as const causes the [] operator to break? I get this compiler error (gcc 4.2) when I use const:

error: no match for ‘operator[]’ in ‘map[name]’

Here's the function prototype:

void func(const char ch, std::string &str, const std::map<std::string, std::string> &map);

And, I should mention that there is no problem when I remove the const keyword in front of std::map.

If I've been instructed correctly, the [] operator will actually insert a new pair into the map if it doesn't find the key, which would of course explain why this happens, but I can't imagine that this would ever be acceptable behavior.

If there is a better method, like using find instead of [], I'd appreciate it. I can't seem to get find to work either though... I receive const mismatched iterator errors.

Thanks.

解决方案

Yes you can't use operator[]. Use find, but note it returns const_iterator instead of iterator:

std::map<std::string, std::string>::const_iterator it;
it = map.find(name);
if(it != map.end()) {
    std::string const& data = it->second;
    // ...
}

It's like with pointers. You can't assign int const* to int*. Likewise, you can't assign const_iterator to iterator.

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