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How to iterate over a STL map full of strings in C++

Posted by: admin November 29, 2017 Leave a comment

Questions:

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: no match for call to ‘(std::basic_string, std::allocator >) ()’”.

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?

Thank you!

Answers:

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) {
    ...
}

Questions:
Answers:
  1. Don’t write a toString() method. This is not Java. Implement the stream operator for your class.

  2. 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.

  3. 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& operator<<(std::ostream& str,something const& data)
{
    data.print(str)
    return str;
}

void something::print(std::ostream& 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 << 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<std::string>(bob);
}

The details that need to be filled in.

class somthing
{
    typedef std::map<std::string,std::string>    DataMap;
    struct PrintData
    {
         PrintData(std::ostream& str): m_str(str) {}
         void operator()(DataMap::value_type const& data) const
         {
             m_str << value.first << "=" << value.second << "\n";
         }
         private:  std::ostream& m_str;
    };
    DataMap    table;
    public:
        void something::print(std::ostream& str);
};

Questions:
Answers:

Change your append calls to say

...append(iter->first)

and

... append(iter->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->append(...)

Be aware that this will construct the string on the stack, then copy it into the return variable. This has performance implications.

Questions:
Answers:

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->first()

to

iter->first

Ditto with iter->second.

Questions:
Answers:

iter->first and iter->second are variables, you are attempting to call them as methods.

Questions:
Answers:

Use:

std::map<std::string, std::string>::const_iterator

instead:

std::map<std::string, std::string>::iterator

Questions:
Answers:

in c++11 you can use

for ( auto iter : table ) {
     key=iter->first();
     value=iter->second();
}

Questions:
Answers:

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.

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