Confused about constructors

This is a discussion on Confused about constructors within the C++ Programming forums, part of the General Programming Boards category; I am trying to figure out how to modify this program so the file is read from the constructor: Code: ...

  1. #1
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    Confused about constructors

    I am trying to figure out how to modify this program so the file is read from the constructor:

    Code:
    #include <iostream> 
    #include <string> 
    #include <fstream> 
    #include <vector>
    using namespace std; 
    
    
    class Scramble { 
          private: 
                   vector<string> _word;
                   bool Check (string T, string S); 
          public: 
                  Scramble(vector<string> words){  
                  _word=words; 
                  
    } 
    void Descramble (string Scrambled); 
    }; 
    //******************************************
    void Scramble::Descramble(string Scrambled)
    { 
          bool flag=false; 
          for(int i=0;i<_word.size();i++)
          { 
                  if(_word[i].size()==Scrambled.size())
                  {
                       if(Check(Scrambled,_word[i])) 
                       {
                            cout << "The word is: " << _word[i]<< endl; 
                            flag=true;}
                  }
          }
          if (!flag)
          {
                  cout<< "The word: " << Scrambled <<" is not found"<< endl; 
                  return;
          }
    }
    //******************************************
    bool Scramble::Check(string T, string S)
    { 
          int n=T.size(); 
          int Found=0; 
          string temp=S; 
          for (int i=0; i<n; i++) 
          { 
                  bool flag = true; 
                  for (int j=0; j < n && flag;j++) 
                  if(T[i]== temp[j])
                  { 
                       temp[j]=' ';
                       flag=false;Found ++;
                  } 
          }
          if (Found==n)
                  return true; 
          else 
                  return false;
    } 
    //******************************************
    int main(){ 
          cout << "Welcome to the Scramble Game!\n\n";
          while (true)
          {
                  int play;
                  cout << "\nWould you like to play? \nEnter 1 for Yes or 2 for No:\n";
                  cin >> play;
                  if (play==1)
                  {
                       cout <<"\nPlease enter the scrambled word: ";
                       string Scrambled ;
                       cin >> Scrambled; 
                       ifstream file("dictionary.txt");
                   if(!file)
                       {
    	                    cout <<"\nError! The file couldn't be opened\n";
                       }
                        vector<string> word_list;
                       string word;
                       while(file >> word)
    	               {
                            word_list.push_back(word);
                       }
                           Scramble test(word_list); 
                       test.Descramble(Scrambled);
                       file.close();
                       cin.get(); cin.get();
                      
                      
                  
                  } 
                  else if (play==2)
                  {
                       cout << "\n\nGoodbye!\n\n";
                       cin.get(); cin.get();
                       return 0;
                  }
                  else
                  {
                       cout << "\nInvalid entry. Try again.\n\n";
                       cin.get(); cin.get();
                  }
          }
    }
    Would I just move the code for the "play" option to the Scramble class?

  2. #2
    Captain - Lover of the C
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    This code would need to be in the constructor and you would need to pass the filename to it.
    Code:
    ifstream file("dictionary.txt");
    if(!file)
    {
        cout <<"\nError! The file couldn't be opened\n";
    }
    vector<string> word_list;
    string word;
    while(file >> word)
    {
        word_list.push_back(word);
    }
    Don't quote me on that... ...seriously

  3. #3
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    The problem with adding that code to the constructor is it makes it difficult to return an error. You have to either throw an exception (in which case the class object is never created), or set a member variable to indicate that initialization failed and hope the creator of the class checks that member before using the class.

    It would be better to add the code to a new member function which could return an error if the file failed to open.

  4. #4
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    I think it's generally better to let the constructor open the file. You need to ask, does the class make any sense if the file cannot be opened? In this case, no, because then you have no dictionary to check against.

    So if the file fails to open in the constructor, you can throw an exception. You can also indicate error with a flag, or just continue with an empty dictionary. The exception might be the best choice, since the dictionary file should really always be there, but if you haven't learned exceptions yet then perhaps either of the other two options or something else would be fine.

  5. #5
    C++ Witch laserlight's Avatar
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    I think it's generally better to let the constructor open the file. You need to ask, does the class make any sense if the file cannot be opened? In this case, no, because then you have no dictionary to check against.
    In that case, one could attempt to open the file. If successful, one then goes on to construct the object, passing the input/output stream opened to the constructor, and then work with the object. Otherwise, one would handle the error.
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  6. #6
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    Agreed. There are many options. I guess I'm just against having a separate member function to handle the file. The class should be usable once it's been constructed rather than having a second function that must be called to initialize.

  7. #7
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Daved View Post
    Agreed. There are many options. I guess I'm just against having a separate member function to handle the file. The class should be usable once it's been constructed rather than having a second function that must be called to initialize.
    In other words, "RAII, baby."

  8. #8
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    Thanks for all the answers, but I need a little more help witht the actual syntax. If I try Brad's suggestion and move that piece of code, how do I keep the program flowing? Do I call the constructor like a function?

  9. #9
    C++ Witch laserlight's Avatar
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    Do I call the constructor like a function?
    You would write something like:
    Code:
    Scramble scramble("dictionary.txt");
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  10. #10
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    When I try "Scramble scramble("dictionary.txt");"

    I get these errors :

    no matching function for call to `Scramble::Scramble(const char[15])'

    candidates are: Scramble::Scramble(const Scramble&)

    Scramble::Scramble(std::vector<std::string, std::allocator<std::string> >)

  11. #11
    C++ Witch laserlight's Avatar
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    Read Brad0407's suggestion again: "This code would need to be in the constructor and you would need to pass the filename to it."
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  12. #12
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    In other words, you need to create a new constructor that takes a filename.

  13. #13
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    I know this probably sounds stupid, but how do I pass the filename to the constructor? I can't seem to find any examples! And why do I need 2 constructors? Can't I just add parameters to the one I am already using?

  14. #14
    C++ Witch laserlight's Avatar
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    Can't I just add parameters to the one I am already using?
    You can do that too, of course. It may even be better.
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  15. #15
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    Ok, so how do I go about that? What would that parameter be? And why couldn't I just open the file in the constructor rather than passing it from main and then opening it?

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