转载 一个与map下标操作有关的编译错误

        来， 看程序：

 
 

   
   

    
    

   
   
   
   

    
    

     
     #include <iostream>
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     #include <string>
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     #include <map>
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     using 
     
     namespace 
     
     std;
    
    
   
   

   
   

    
    

   
   
   
   

    
    
 
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     class A
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     {
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     public:
    
    
   
   

   
   

    
    

   
   
   
   

    
    
	
     
     map<
     
     string, 
     
     string> m_map;	
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     };
    
    
   
   

   
   

    
    

   
   
   
   

    
    
 
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     void test(const A &a)
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     {
    
    
   
   

   
   

    
    

   
   
   
   

    
    
	
     
     string s = a.m_map[
     
     "hello"];
    
    
   
   

   
   

    
    

   
   
   
   

    
    
	
     
     cout << s << 
     
     endl;
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     }
    
    
   
   

   
   

    
    

   
   
   
   

    
    
 
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     int main()
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     {
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     	A a;
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     	a.m_map[
     
     "hello"] = 
     
     "world";
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     	test(a);
    
    
   
   

   
   

    
    

   
   
   
   

    
    
 
    
    
   
   

   
   

    
    

   
   
   
   

    
    
	
     
     return 
     
     0;
    
    
   
   

   
   

    
    

   
   
   
   

    
    

     
     }
    
    
   
   
 
 

      g++编译错误如下：

main.cpp: In function ‘void test(const A&)’:
main.cpp:14: error: passing ‘const std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >’ as ‘this’ argument of ‘_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _Tp = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _Compare = std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, _Alloc = std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >]’ discards qualifiers

     

       原因： 在test中， a的值不能变， 而a.m_map["hello"]是一个可读写操作， 所以有问题。  还是用const_iterator吧， 先查找"hello",  再访问it->second