Question

Consider a std::map<const char *, MyClass*>.

How do I access a member (variable or function) of the MyClass object pointed to by the map?

// assume MyClass has a string var 'fred' and a method 'ethel'
std::map<const char*, MyClass*> MyMap;

MyMap[ "A" ] = new MyClass;
MyMap.find( "A" )->fred = "I'm a Mertz";  // <--- fails on compile
MyMap.find( "A" )->second->fred = "I'm a Mertz";  // <--- also fails

---

EDIT — per Xeo’s suggestion

I posted dummy code. Here is the real code.

// VarInfo is meta-data describing various variables, type, case, etc.
std::map<std::string,VarInfo*> g_VarMap; // this is a global 

int main( void )
{ 
   // ........ g_VarMap["systemName"] = new VarInfo; 
   g_VarMap.find( "systemName" ).second->setCase( VarInfo::MIXED, VarInfo::IGNORE ); 
   // ..... 
} 

errors were:

struct std::_Rb_tree_iterator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, VarInfo*> >’ has no member named ‘second’
Field 'second' could not be resolved Semantic Error make: *** [src/ACT_iod.o] Error 1 C/C++ Problem
Method 'setCase' could not be resolved Semantic Error – 

Answer 1

std::map stores types internally as a std::pair, and std::map::find, returns an iterator. So, to access members of your class, you have to go through the iterator, which presents the key_type as first, and the value_type as second. Also, as others have stated, you should probably not be using const char* as your key_type. Here’s a short example.

#include <string>
#include <map>
#include <iostream>

struct T
{
   T(int x, int y) : x_(x), y_(y)
   {}

   int x_, y_;
};

int main()
{
   typedef std::map<std::string, T> map_type;
   map_type m;

   m.insert(std::make_pair("0:0", T(0,0)));
   m.insert(std::make_pair("0:1", T(0,1)));
   m.insert(std::make_pair("1:1", T(1,1)));

   // find the desired item (returns an iterator to the item
   // or end() if the item doesn't exist.
   map_type::const_iterator t_0_1 = m.find("0:1");

   if(m.end() != t_0_1)
   {
      // access via the iterator (a std::pair) with 
      // key stored in first, and your contained type
      // stored in second.
      std::cout << t_0_1->second.x_ << ':' << t_0_1->second.y_ << '\n';
   }

   return 0;
}