About iterator and const problem in C++

class LogManager {
  private:
    mutable mapManagerMutex mapMutex;

    typedef std::map<std::string, LogStorage*> FileNameToStorageClass;
    FileNameToStorageClass  m_mapFileNameToLogStrg;

  public:
    int write( const string& strFileName, char* text ) const;

};

int LogManager::write( const string &strFileName, char* text ) const
{
   mapManagerMutex::scoped_lock lock(mapMutex);
   FileNameToStorageClass::iterator iter;
   iter = m_mapFileNameToLogStrg.find(strFileName);
   if(iter != m_mapFileNameToLogStrg.end())
   {
     // do some thing.
   }
   else
   {
     return -1;
   }
   return 0;
}

Above code compiles if i don't have const at end of write function. If i add const at end 开发者_如何转开发i am getting following error

D:/LogManager.cpp:133: error: no match for 'operator=' in 'iter = ((const RtsInfrastructure::RtsCommon::Diagnostics::LogManager*)this)-
cc: C:/QNX650/host/win32/x86/usr/lib/gcc/i486-pc-nto-qnx6.5.0/4.4.2/cc1plus caught signal 1

Does any body know why we are seeing this?

Because m_mapFileNameToLogStrg is treated as a const member inside the const member function. This is because the type of this pointer is 'LogManager const *'. Then how can the iterator iter be non-const?

If iter is non-const, it could be used to modify the member m_mapFileNameToLogStrg (which is treated as const) thereby violating constness.

So here are a few options:

a) make the member function non-const (write member function being const??)

b) make the string data member mutable.

c) use const_iterator as suggested by Luca Martini

Use FileNameToStorageClass::const_iterator instead of FileNameToStorageClass::iterator.

code compiles if i don't have const at end of write function

That tells you that you're doing some non-const operation on a member, such as calling a non-const function. Given the only member you're doing anything to is m_mapFileNameToLogStrg, check whether its find() and end() are const. Then, as Luca says, the const version of find() should return a const iterator. If you (or your company) have implemented these container types themselves, its quite possible that they don't even have proper const iterators, so you should add them.