How to iterate over a std map full of strings in C

Question

I have the following issue related to iterating over an associative array of strings defined using std  map   -- snip -- class something             private        std  map lt std  string  std  string gt  table            In the constructor I populate table with pairs of string keys associated to string data  Somewhere else I have a method toString that returns a string object that contains all the keys and associated data contained in the table object as key data format    std  string something  toString              std  map lt std  string  std  string gt   iterator iter          std  string  strToReturn   new std  string               for  iter   table begin    iter    table end    iter                 strToReturn- gt append iter- gt first                strToReturn- gt append                  strToRetunr- gt append iter- gt second                                                  When I m trying to compile I get the following error   error   error  no match for call to     std  basic string lt char      std  char traits lt char gt   std  allocator lt char gt   gt             Could somebody explain to me what is missing  what I m doing wrong   I only found some discussion about a similar issue in the case of hash map where the user has to define a hashing function to be able to use hash map with std  string objects  Could be something similar also in my case

User · Accepted Answer

Your main problem is that you are calling a method called first() in the iterator. What you are meant to do is use the property called first:

...append(iter->first) rather than ...append(iter->first())

As a matter of style, you shouldn't be using new to create that string.

std::string something::toString() 
{
        std::map<std::string, std::string>::iterator iter;
        std::string strToReturn; //This is no longer on the heap

        for (iter = table.begin(); iter != table.end(); ++iter) {
           strToReturn.append(iter->first); //Not a method call
           strToReturn.append("=");
           strToReturn.append(iter->second);
           //....
           // Make sure you don't modify table here or the iterators will not work as you expect
        }
        //...
        return strToReturn;
}

edit: facildelembrar pointed out (in the comments) that in modern C++ you can now rewrite the loop

for (auto& item: table) {
    ...
}

User · Answer

Use   std  map lt std  string  std  string gt   const iterator   instead   std  map lt std  string  std  string gt   iterator

User · Answer

In c  11 you can use   for   auto iter   table          key iter- gt first       value iter- gt second

User · Answer

Another worthy optimization is the c str     member of the STL string classes  which returns an immutable null terminated string that can be passed around as a LPCTSTR  e  g   to a custom function that expects a LPCTSTR  Although I haven t traced through the destructor to confirm it  I suspect that the string class looks after the memory in which it creates the copy

User · Answer

Note that the result of dereferencing an std  map  iterator is an std  pair  The values of first and second are not functions  they are variables   Change   iter- gt first     to  iter- gt first   Ditto with iter- gt second

User · Answer

iter- gt first and iter- gt second are variables  you are attempting to call them as methods

User · Answer

Don t write a toString   method  This is not Java  Implement the stream operator for your class  Prefer using the standard algorithms over writing your own loop  In this situation  std  for each   provides a nice interface to what you want to do  If you must use a loop  but don t intend to change the data  prefer const iterator over iterator  That way  if you accidently try and change the values  the compiler will warn you    Then   std  ostream amp  operator lt  lt  std  ostream amp  str something const amp  data        data print str      return str     void something  print std  ostream amp  str  const       std  for each table begin   table end   PrintData str        Then when you want to print it  just stream the object   int main         something    bob      std  cout  lt  lt  bob      If you actually need a string representation of the object  you can then use lexical cast   int main         something    bob       std  string  rope   boost  lexical cast lt std  string gt  bob       The details that need to be filled in   class somthing       typedef std  map lt std  string std  string gt     DataMap      struct PrintData                PrintData std  ostream amp  str   m str str              void operator   DataMap  value type const amp  data  const                         m str  lt  lt  data first  lt  lt       lt  lt  data second  lt  lt    n                       private   std  ostream amp  m str             DataMap    table      public          void something  print std  ostream amp  str

User · Answer

Change your append calls to say     append iter- gt first    and       append iter- gt second    Additionally  the line  std  string  strToReturn   new std  string        allocates a string on the heap  If you intend to actually return a pointer to this dynamically allocated string  the return should be changed to std  string    Alternatively  if you don t want to worry about managing that object on the heap  change the local declaration to   std  string strToReturn        and change the  append  calls to use reference syntax     strToReturn append        instead of   strToReturn- gt append        Be aware that this will construct the string on the stack  then copy it into the return variable  This has performance implications

[c++] How to iterate over a std::map full of strings in C++

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