Thread: Deciphering a constructor.

Hello,

I'm trying to get started with c++ by modifying some code from a K&P book that parses CSV data via cin, to read the data from a file. I am having great difficulty understanding the class constructor as it is implemented:

Code:

Csv(istream& fin = cin, string sep = ",") : fin(fin), fieldsep(sep) {}

I thought it would be as straightforward as replacing cin by a filepath but it doesn't work. I don't understand what the code after the colon is doing, especially fin(fin). The variables are defined as private:

Code:

  private:
    istream& fin;           // Input file pointer.
    string line;            // Input line.
    vector<string> field;   // Field strings.
    int nfield;             // Number of fields.
    string fieldsep;        // Separator characters.

Could someone please break this constructor down and explain it. I have not seen this type of constructor anywhere else.

Thanks

The code behind the colon is called member initializer list.
Constructors and member initializer lists - cppreference.com

It also could be written like:

Code:

Csv(istream& fin = cin, string sep = ",")
{
  this->fin = fin;
  fieldsep = sep;
}

I thought it would be as straightforward as replacing cin by a filepath but it doesn't work.

You probably shouldn't be playing with the class definition, just call the constructor with the proper values.

Code:

    // It is probably being called like:
    // Csv from_console;

    // To use a file you will need to pass an open file description to the constructor along with the delimiter you want to use.
    ifstream from_file("Your_csv_file_name");
    if(!from_file)
    {
        // Take some action if the file failed to open.
    }

    // Create an instance of Csv using the from_file stream and a comma as the delimiter. 
    Csv file(from_file, ',');

Don't try to translate c to C++. They are total different languages.
A simple demo how to read a file line by line

Code:

#include <iostream>
#include <string>
#include <cstdlib> // perror, errno
#include <fstream>

int main()
{
  const std::string filename = "FileName"; 
  std::ifstream src(filename); // src(filename.c_str()) for oldeer compiler; 

  if (!src.good())
  {
    perror("Can't open file");
    return errno;
  }
  std::string line;
  while (std::getline(src, line)) // read's either till '\n' oe end of stream
  {
    // use line
  }
}

That last line is causing problems(no matching function call).

I've added the fstream library. I'm still missing something.

Even so, I at least have some direction now so many thanks to you both.

You'll need to post more content, a small complete program would be best. Also if you're getting error messages you need to post the complete error message, all of them, exactly as they appear in your development environment.

By the way I changed the name of the instance in my snippet you'll need to use the proper names not just cut and past my snippet.

Lastly why the "MACROS"? They should be avoided in C++ programs, prefer const qualified variables instead.

Originally Posted by OldGuy2

Don't try to translate c to C++. They are total different languages.
A simple demo how to read a file line by line

Thanks again. I've abandoned trying to adapt that example and managed to implement a solution using the Boost Tokenizer. I understand your code, and it is essentially how I read each line in. There was just too much in that K&P example that I couldn't understand fully enough. I didn't want to use Boost but at least I have understood the examples from the library.

Regards

Originally Posted by jimblumberg

Lastly why the "MACROS"? They should be avoided in C++ programs, prefer const qualified variables instead.

I've converted the macros to const vars and read-up on the pros and cons. Thanks for the tip.