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<std::string, std::string> 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<std::string, std::string>::iterator iter;
        std::string* strToReturn = new std::string("");

        for (iter = table.begin(); iter != table.end(); iter++) {
           strToReturn->append(iter->first());
           strToReturn->append('=');
           strToRetunr->append(iter->second());
           //....
        }
       //...
}

When I’m trying to compile I get the following error:

error: "error: no match for call to ‘(std::basic_string<char,
    std::char_traits<char>, std::allocator<char> >) ()’".

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?

Answer 1

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) {
    ...
}