Question

I have created a class and in the constructor I do something like:

MyClass(string file)
{
    ifstream str;
    str.open (file, ifstream::in);

    // initialize class variables based on values from file

    str.close();
}

How would I know though if there was an error while reading the file and the values were not initialized correctly? Is the above the wrong way to do it? How else could I proceed if I want my variables to be initialized from a file in the constructor?

Edit, to clarify: What I am asking is in a statement like:

MyClass myclass("path/to/my/file.txt");

how could I know that everything was initialized correctly?

Answer 1

One possibility is to use exceptions:

...
ifstream str;
str.exceptions(std::ifstream::failbit);
// continue as before

Then, if your constructor throws an exception, you know something didn’t work.

Another possibility is to detect the failure in the constructor and set a state variable which can be examined by the calling code.

Here are both solutions:

#include <iostream>
#include <exception>
#include <string>
#include <fstream>

struct MyClass1 {
  int i;
  MyClass1(const char * iniFile) {
    std::ifstream in;
    in.exceptions(std::ifstream::failbit);
    in.open(iniFile);
    in >> i;
  }
};

struct MyClass2 {
  int i;
  bool isValid;
  MyClass2(const char * iniFile) {
    try {
      std::ifstream in;
      in.exceptions(std::ifstream::failbit);
      in.open(iniFile);
      in >> i;
      isValid = true;
    } catch(std::ios_base::failure &fail) {
      isValid = false;
    }
  }
};

int main () {
  try {
    MyClass1 mc1("somefile.txt");
  } catch(std::exception& fail) {
    std::cout << "oops 1\n";
  }

  MyClass2 mc2("somefile.txt");
  if(!mc2.isValid) {
    std::cout << "oops 2\n";
  }
}

Assuming that somefile.txt does not exist, that should print two lines of “oops”.

Finally, a third possibility is to ensure that the constructor can never fail. In your case, you could provide default values in the catch block.