CODEWITHSUNDEEP
c++file-iooperators
bruno
bruno

Reputation: 2882

Reading file until blank line

After reading Jerry Coffin's answer on this question I copy-pasted his code into my editor, and after some minor edits it compiled and run like it should.

Here is the code after the changes:

 #include <iostream>
 #include <string>
 #include <istream>
 #include <fstream>

class non_blank {
private:
 std::string data_;

 friend std::istream& operator>> (std::istream &is, non_blank &n) {
  std::getline(is, n.data_);

  if (n.data_.length() == 0) {
   is.setstate(std::ios::failbit);
  }
  return is;
 }

public:
 operator std::string() const {
  return data_;
 }
};


int main(int, char *[]) {

 non_blank line;
 std::ifstream ifs("teste.txt");

 while(ifs >> line) {
  //std::cout << line; <----- error
  std::string s = line;
  std::cout << s << std::endl;
 }

 return 0;
}

Here is the error I got:

..\main.cpp: In function `std::istream& operator>>(std::istream&, non_blank&)':
..\main.cpp:21: error: invalid use of non-static data member `non_blank::data_'
..\main.cpp:26: error: from this location

Upvotes: 2

Views: 2886

Answers (1)

John Zwinck
John Zwinck

Reputation: 249582

Shouldn't I be able to use a variable of the type non_blank anywhere I would use a std::string? Isn't it the porpuse of the cast operator?

Not quite. If the compiler sees you doing something that it knows requires a std::string, it can call your conversion operator to get one. But in the case of the ostream operator <<, it doesn't have a single specific function to call, but rather quite a lot of them, all different and none matching precisely the actual type you mean to print. So it lists a whole bunch of candidates, none of which is a strong enough match. You need to define an ostream operator << for your type in order to make it print as it should.

As for your operator >>, you should make it not be a member of your class. Declare it as a friend within the class declaration if you must, but write the function itself outside.

Upvotes: 1

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