[15] Input/output via <iostream.h> and <stdio.h>
(Part of C++ FAQ Lite, Copyright © 1991-99, Marshall Cline, cline@parashift.com)


FAQs in section [15]:


[15.1] Why should I use <iostream.h> instead of the traditional <stdio.h>?

Increase type safety, reduce errors, improve performance, allow extensibility, and provide subclassability.

printf() is arguably not broken, and scanf() is perhaps livable despite being error prone, however both are limited with respect to what C++ I/O can do. C++ I/O (using << and >>) is, relative to C (using printf() and scanf()):

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[15.2] Why does my program go into an infinite loop when someone enters an invalid input character? UPDATED!

[Recently added return type to main() (on 10/99). Click here to go to the next FAQ in the "chain" of recent changes.]

For example, suppose you have the following code that reads integers from cin:

    #include <iostream.h>
    
    int main()
    {
      cout << "Enter numbers separated by whitespace (use -1 to quit): ";
      int i = 0;
      while (i != -1) {
        cin >> i;        
// BAD FORM — See comments below
        cout << "You entered " << i << '\n';
      }
    }

The problem with this code is that it lacks any checking to see if someone entered an invalid input character. In particular, if someone enters something that doesn't look like an integer (such as an 'x'), the stream cin goes into a "failed state," and all subsequent input attempts return immediately without doing anything. In other words, the program enters an infinite loop; if 42 was the last number that was successfully read, the program will print the message You entered 42 over and over.

An easy way to check for invalid input is to move the input request from the body of the while loop into the control-expression of the while loop. E.g.,

    #include <iostream.h>
    
    int main()
    {
      cout << "Enter a number, or -1 to quit: ";
      int i = 0;
      while (cin >> i) {    
// GOOD FORM
        if (i == -1) break;
        cout << "You entered " << i << '\n';
      }
    }

This will cause the while loop to exit either when you hit end-of-file, or when you enter a bad integer, or when you enter -1.

(Naturally you can eliminate the break by changing the while loop expression from while (cin >> i) to while ((cin >> i) && (i != -1)), but that's not really the point of this FAQ since this FAQ has to do with iostreams rather than generic structured programming guidelines.)

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[15.3] How does that funky while (cin >> foo) syntax work? UPDATED!

[Recently corrected a serious bug: changed operator bool() to operator void*(); thanks to Rajinikanth Sivalingam (on 7/99). Click here to go to the next FAQ in the "chain" of recent changes.]

See the previous FAQ for an example of the "funky while (cin >> foo) syntax."

The expression (cin >> foo) calls the appropriate operator>> (for example, it calls the operator>> that takes an istream on the left and, if foo is of type int, an int& on the right). The istream operator>> functions return their left argument by convention, which in this case means it will return cin. Next the compiler notices that the returned istream is in a boolean context, so it converts that istream into a boolean.

To convert an istream into a boolean, the compiler calls a member function called istream::operator void*(). This returns a void* pointer, which is in turn converted to a boolean (NULL becomes false, any other pointer becomes true). So in this case the compiler generates a call to cin.operator void*(), just as if you had casted it explicitly such as (void*)cin.

The operator void*() cast operator returns some non-NULL pointer if the stream is in a good state, or NULL if it's in a failed state. For example, if you read one too many times (e.g., if you're already at end-of-file), or if the actual info on the input stream isn't valid for the type of foo (e.g., if foo is an int and the data is an 'x' character), the stream will go into a failed state and the cast operator will return NULL.

The reason operator>> doesn't simply return a bool (or void*) indicating whether it succeeded or failed is to support the "cascading" syntax:

      cin >> foo >> bar;

The operator>> is left-associative, which means the above is parsed as:

      (cin >> foo) >> bar;

In other words, if we replace operator>> with a normal function name such as readFrom(), this becomes the expression:

      readFrom( readFrom(cin, foo), bar);

As always, we begin evaluating at the innermost expression. Because of the left-associativity of operator>>, this happens to be the left-most expression, cin >> foo. This expression returns cin (more precisely, it returns a reference to its left-hand argument) to the next expression. The next expression also returns (a reference to) cin, but this second reference is ignored since it's the outermost expression in this "expression statement."

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[15.4] Why does my input seem to process past the end of file? UPDATED!

[Recently strengthened the message with another example (on 10/99). Click here to go to the next FAQ in the "chain" of recent changes.]

Because the eof state may not get set until after a read is attempted past the end of file. That is, reading the last byte from a file might not set the eof state. E.g., suppose the input stream is mapped to a keyboard -- in that case it's not even theoretically possible for the C++ library to predict whether or not the character that the user just typed will be the last character.

For example, the following code might have an off-by-one error with the count i:

    int i = 0;
    while (! cin.eof()) {   
// WRONG! (not reliable)
      cin >> x;
      ++i;
      
// Work with x ...
    }

What you really need is:

    int i = 0;
    while (cin >> x) {      
// RIGHT! (reliable)
      ++i;
      
// Work with x ...
    }

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[15.5] Why is my program ignoring my input request after the first iteration?

Because the numerical extractor leaves non-digits behind in the input buffer.

If your code looks like this:

    char name[1000];
    int age;
    
    for (;;) {
      cout << "Name: ";
      cin >> name;
      cout << "Age: ";
      cin >> age;
    }

What you really want is:

    for (;;) {
      cout << "Name: ";
      cin >> name;
      cout << "Age: ";
      cin >> age;
      cin.ignore(INT_MAX, '\n');
    }

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[15.6] How can I provide printing for my class Fred? UPDATED!

[Recently added return type to main() (on 10/99). Click here to go to the next FAQ in the "chain" of recent changes.]

Use operator overloading to provide a friend left-shift operator, operator<<.

    #include <iostream.h>
    
    class Fred {
    public:
      friend ostream& operator<< (ostream& o, const Fred& fred);
      
// ...
    private:
      int i_;    
// Just for illustration
    };
    
    ostream& operator<< (ostream& o, const Fred& fred)
    {
      return o << fred.i_;
    }
    
    int main()
    {
      Fred f;
      cout << "My Fred object: " << f << "\n";
    }

We use a non-member function (a friend in this case) since the Fred object is the right-hand operand of the << operator. If the Fred object was supposed to be on the left hand side of the << (that is, myFred << cout rather than cout << myFred), we could have used a member function named operator<<.

Note that operator<< returns the stream. This is so the output operations can be cascaded.

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[15.7] How can I provide input for my class Fred? UPDATED!

[Recently added return type to main() (on 10/99). Click here to go to the next FAQ in the "chain" of recent changes.]

Use operator overloading to provide a friend right-shift operator, operator>>. This is similar to the output operator, except the parameter doesn't have a const: "Fred&" rather than "const Fred&".

    #include <iostream.h>
    
    class Fred {
    public:
      friend istream& operator>> (istream& i, Fred& fred);
      
// ...
    private:
      int i_;    
// Just for illustration
    };
    
    istream& operator>> (istream& i, Fred& fred)
    {
      return i >> fred.i_;
    }
    
    int main()
    {
      Fred f;
      cout << "Enter a Fred object: ";
      cin >> f;
      
// ...
    }

Note that operator>> returns the stream. This is so the input operations can be cascaded and/or used in a while loop or if statement.

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[15.8] How can I provide printing for an entire hierarchy of classes?

Provide a friend operator<< that calls a protected virtual function:

    class Base {
    public:
      friend ostream& operator<< (ostream& o, const Base& b);
      
// ...
    protected:
      virtual void print(ostream& o) const;
    };
    
    inline ostream& operator<< (ostream& o, const Base& b)
    {
      b.print(o);
      return o;
    }
    
    class Derived : public Base {
    protected:
      virtual void print(ostream& o) const;
    };

The end result is that operator<< acts as if it was dynamically bound, even though it's a friend function. This is called the Virtual Friend Function Idiom.

Note that derived classes override print(ostream&) const. In particular, they do not provide their own operator<<.

Naturally if Base is an ABC, Base::print(ostream&) const can be declared pure virtual using the "= 0" syntax.

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[15.9] How can I "reopen" cin and cout in binary mode under DOS and/or OS/2?

This is implementation dependent. Check with your compiler's documentation.

For example, suppose you want to do binary I/O using cin and cout. Suppose further that your operating system (such as DOS or OS/2) insists on translating "\r\n" into "\n" on input from cin, and "\n" to "\r\n" on output to cout or cerr.

Unfortunately there is no standard way to cause cin, cout, and/or cerr to be opened in binary mode. Closing the streams and attempting to reopen them in binary mode might have unexpected or undesirable results.

On systems where it makes a difference, the implementation might provide a way to make them binary streams, but you would have to check the manuals to find out.

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[15.10] Why can't I open a file in a different directory such as "..\test.dat"? UPDATED!

[Recently added return type to main() (on 10/99). Click here to go to the next FAQ in the "chain" of recent changes.]

Because "\t" is a tab character.

You should use forward slashes in your filenames, even on an operating system that uses backslashes such as DOS, Windows, OS/2, etc. For example:

    #include <iostream.h>
    #include <fstream.h>
    
    int main()
    {
      #if 1
        ifstsream file("../test.dat");     
// RIGHT!
      #else
        ifstsream file("..\test.dat");     
// WRONG!
      #endif
    
      
// ...
    }

Remember, the backslash ("\") is used in string literals to create special characters: "\n" is a newline, "\b" is a backspace, and "\t" is a tab, "\a" is an "alert", "\v" is a vertical-tab, etc. Therefore the file name "\version\next\alpha\beta\test.dat" is interpreted as a bunch of very funny characters; use "/version/next/alpha/beta/test.dat" instead, even on systems that use a "\" as the directory separator such as DOS, Windows, OS/2, etc.

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Revised Oct 15, 1999