reference to const static object variable

[Bob Doe]

I have a const static object. What is the right syntax to get a
reference to the std::vector within the std::map variable?:

class MyObj
{
public:
...
std::map<std::string,
std::vector<std::string> > l_;
};

static MyObj obj;

int main()
{
...
const std::vector<std::string> &regexList = obj.l_["key"]; //
compiler error
...
};

 

[Bob Doe]

sorry,

The example should have been:

class MyObj
{
public:
MyObj()
{
std::vector<std::string> t;
t.push_back("zzz");
l_["a"] = t;
}

std::map<std::string,
std::vector<std::string> > l_;
};

const static MyObj obj;

int main()
{
const std::vector<std::string> &regexList = obj.l_["key"];
}


-----
I want to maintain the const-ness of MyObj, and have read access to
the std::vector within std::map...

and the error message says:

passing `const

,vector<basic_string<char,string_char_traits<char>,__default_alloc_template<false, 0>
,allocator<basic_string<char,string_char_traits<char>,__default_alloc_template<false, 0> > >
,less<basic_string<char,string_char_traits<char>,__default_alloc_template<false, 0> >
,allocator<vector<basic_string<char,string_char_traits<char>,__default_alloc_template<false, 0>
,allocator<basic_string<char,string_char_traits<char>,__default_alloc_template<false,

0> > > > > >' as `this' argument of `class

,allocator<basic_string<char,string_char_traits<char>,__default_alloc_template<false,

0> > > > &

,vector<basic_string<char,string_char_traits<char>,__default_alloc_template<false, 0>
,allocator<basic_string<char,string_char_traits<char>,__default_alloc_template<false, 0> > >
,less<basic_string<char,string_char_traits<char>,__default_alloc_template<false, 0> >
,allocator<vector<basic_string<char,string_char_traits<char>,__default_alloc_template<false, 0>
,allocator<basic_string<char,string_char_traits<char>,__default_alloc_template<false,

0> > > > > >:perator [](const string &)' discards qualifiers


Which I don't understand.

const static MyObj obj;

I have a const static object.  What is the right syntax to get a
reference to the std::vector within the std::map variable?:

class MyObj
{
   public:
     ...
     std::map<std::string,
                  std::vector<std::string> > l_;
};

 static MyObj obj;

int main()
{
  ...
  const std::vector<std::string> &regexList = obj.l_["key"];  //
compiler error
  ...
};

Did you care to read the error message? What did it say?

 

[James Kanze]

sorry,

The example should have been:

class MyObj
{
   public:
      MyObj()
      {
         std::vector<std::string> t;
         t.push_back("zzz");
         l_["a"] = t;
      }

      std::map<std::string,
               std::vector<std::string> > l_;
};

const static MyObj obj;

int main()
{
   const std::vector<std::string> &regexList = obj.l_["key"];

Have you looked at the documentation of std::map<>perator[]?
What does it do?

Since it modifies the map, you can't use it on a const object.

}

I want to maintain the const-ness of MyObj, and have read
access to the std::vector within std::map...

Then operator[] isn't the function you want. Something like:

::const_iterator

entry = obj.l_[ "key" ] ;
if ( entry != obj.l_.end() ) {
std::vector< std::string > const&
regexList = entry->second ;
// ...
}

maybe. (Or maybe std::map isn't what you really want, except
buried deep in the implementation of something with a more
congenial interface.)

 

[James Kanze]

[snip]

I want to maintain the const-ness of MyObj, and have read
access to the std::vector within std::map...

Then operator[] isn't the function you want.  Something like:

::const_iterator

                        entry = obj.l_[ "key" ] ;

const_iterator entry = obj.find("key");

Yes, obviously.