Input/output via <iostream> and <cstdio>

Part of C++ FQA Lite. To see the original answers, follow the FAQ links.

This section explains the benefits of iostream - a "type-safe" I/O library (which does not mean it will save keystrokes when you type some code).

[15.1] Why should I use <iostream> instead of the traditional <cstdio>?

FAQ: There are four reasons:

FQA: Why should I do this, why should I do that, you ask. What kind of manners are these? Do what you are told. Assignment Number 1 - convert all your evil printf("0x%08xn", x) statements to this:

std::cout << std::hex << std::setfill('0') << std::setw(8) << x << std::dec << std::endl; 

Even if you commit the environmental crime of namespace pollution, adding a using namespace std and removing those pesky std::, the verbosity is still amazing. This achievement is not accidental. It follows from one of the basic flaws in the C++ way of thinking: the "everything is a type" axiom. For example, hex has a special type which hexes streams, and so does every other strange object sent to cout.

The FAQ explains why this thinking is good for you. Here's why the FAQ is wrong:

iostream. The only thing you'll gain from all this extra typing is extra long build cycles and error messages and extra large program image. This is what you get when you shift a file object by an integer.

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

FAQ: Probably you did something like int i; std::cin >> i; without checking for an error. From now on, all attempts to read stuff from std::cin won't actually read anything, because the stream has entered an erroneous state. Presumably, you wrote a program that will still keep trying. Use something like while(std::cin >> i) instead.

FQA: Yep, that's iostream's way to handle input errors. For some reason, it doesn't throw an exception (it's not really bad, because what's really bad is C++ exceptions). But it doesn't return an error code, either, because the overloaded operator has to return its first argument for the "neat" operator call chaining to work. So it has to set an error bit - the dreaded "zombie object" pattern the FAQ condemns so convincingly.

You can then check for errors using if(std::cin) or the like because the object has overloaded operator void*, and void* is convertible to bool. You can also check for end-of-file conditions this way. Actually you can check for error-or-end-of-file conditions. The more operator calls you chain, the less details about the error context are left. Simple, safe, efficient and elegant.

Oh, and iostream has other state bits. For example, std::hex transfers it to hex mode. People frequently forget to undo the effect, which is especially entertaining when they print numbers like 10, which is as good a decimal ten as it is a hexadecimal sixteen. Well, if you found the bug (which for some reason went undetected by the type-safe almighty paranoid C++ compiler), you can use std::dec to get the stream back to decimal mode. Um, actually, it transfers it to decimal mode, which is not necessarily "back", because we don't know which mode it was in before it was hexed. This gets interesting when people print numbers and user-defined objects in the same statement, and count on their hexing to maintain its effect after the user-defined object is printed.

[15.3] How can I get std::cin to skip invalid input characters?

FAQ: Here's an example:

   while ((std::cout << "Give me your credit card number now!!") && !(std::cin >> n)) {
     std::cout << "Don't mess with me, I'm written in C++!!!";
     std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

FQA: Oops, you forgot to test for end of file. If a brave user hits Ctrl-D or whatever it takes to close the standard input with the given terminal, your program will enter an infinite loop. Overloaded operators and error handling are not very compatible.

std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); speaks for itself. You just can't invent it if your "use case" is yourself getting any work done.

[15.4] How does that funky while (std::cin >> foo) syntax work?

FAQ: istream has overloaded operator void*. The compiler calls this operator in boolean contexts (when it expects a condition, for example), because void* can be converted to a boolean. The operator returns NULL when there's nothing left to read, or when a format error occurred previously.

FQA: There's nothing "funky" about it. It is ugly and boring. It's also scary because many people think this is what programming is all about - using complicated syntax to do simple things without even getting them right (how do you tell end-of-file conditions from format errors?).

Why is it operator void*, and not operator bool? Apparently because the compiler implicitly converts booleans to numbers in "numeric contexts" (such as file1+file2), and we don't want that to compile, do we?

But wait, there's more! There's an actual book out there, called "Imperfect C++", arguing that operator void* is not the way to go, either. Because this way, delete file would compile. Surely we don't want it to, do we? I mean, the fact that the statement is completely moronic shouldn't matter. Morones have a right to get an equal opportunity in the exciting world of C++ programming; let's catch of all their errors at compile time. Evil people spread rumors about the problem being undecidable, but we should keep trying.

[15.5] Why does my input seem to process past the end of file?

FAQ: Because the stream doesn't know that you've reached the end of file until you actually try to read past the end of file. For instance, a stream object may read characters which are entered interactively using a keyboard. So it's impossible to tell when it's over.

FQA: That's right, and has nothing to do with iostream - except the fact that iostream handles I/O errors in the silly way discussed above.

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

FAQ: Because non-digits are left in the input buffer. You can ignore them using calls like std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

FQA: Doesn't the fact that there are so many questions about the handling of input format problems ring a bell? This silent treatment of errors is wrong. Stuff like parsing is better handled by languages which have good support for exceptions (built-in strings and containers help, too). If you have to live without exceptions, at least the library should treat errors reasonably. The iostream interface of operator call chains makes error handling as awkward as it gets.

[15.7] Should I end my output lines with std::endl or '\n'?

FAQ: The former has the additional side-effect of flushing the output buffer. Therefore, the latter will probably work faster.

FQA: The latter is also shorter, unless you have a using namespace std. Many people probably ask this question because they think that endl will end the line the way it's normally done at a given platform (say, emit CRLF on Windows and LF on Unix). When endl behaves that way, so does '\n'. You can suppress this behavior when you open the file (pass ios::binary with iostream or the "b" flag with fopen).

[15.8] How can I provide printing for my class Fred?

FAQ: By adding a friend std::ostream& operator<< (std::ostream& o, const Fred& fred);. The operator can't be a member of the class, because we need the object to be the second argument, not the first one.

FQA: Yeah, yeah, C++ and its stupid syntax. Let's forget about it for a moment: the real problem here is semantical, and it starts at the question "How can I provide printing for my class?". The formulation encourages to give an answer in the spirit of "everything is a type", and indeed this is the kind of answer given by the FAQ. Here's the way to print all objects of your class.

But was that really your question? What if you want more than a single output format for your class? For instance, you might want a textual format for people and a binary format for machines. Or there could be several textual formats - even integers can be printed using different bases. Do you really want to develop and maintain the layers of syntax needed to make things like hex and dec sort of work? Why not just define named functions for printing?

The friend thing is also questionable. With all due respect, you might want to print to several different output channels, without creating an adapter class derived from ostream. One simple reason for such heretical behavior is performance - ostream may be too slow for things like real-time logging. Why not create accessors to the data you want to print, so that all the different output methods won't need to be part of your class, which may then have a stable interface? Encapsulation? You've already made your data public - one can read it by printing to a stringstream. Providing accessors violates obfuscation, not encapsulation.

If you abandon the friend part and the operator part, the remaining part of the FAQ's answer - create a global function accepting some kind of stream object and an object of your class and do the printing there - makes a good advice. In particular, it may be better than a method for the same reasons making it better than a friend (however, unlike a friend, a method may be polymorphic, which might be useful).

[15.9] But shouldn't I always use a printOn() method rather than a friend function?

FAQ: Not "always". A method may be useful for a class hierarchy, for example, but in that case it would normally be protected, not public.

Some people think friend violates encapsulation, and therefore prefer to have their operator<< call a public method of the class, even though there's no class hierarchy. Their basic assumption is wrong, so they end up writing extra code that doesn't solve any real problems and confuses people.

FQA: Just to make things clear: a printOn() method is something like void Fred::printOn(std::ostream& o) const.

The FAQ is right about the important thing: creating extra layers of code doing nothing but delegating work to other layers of code is a pesky habit. Many people feel that "good design" means "lots of code" or "lots of layers". Actually, those are some of the meanings of "bad design".

The fact that friend is really needed here to compensate for syntactical limitations of C++, as well as the problems with the whole friend operator approach, are discussed in the previous FAQ. They are not the main theme here. The main theme here is the message to the "designers": listen to the voice of C++ overlords who originally inspired you to create N layers with 0 functionality, and stop.

[15.10] How can I provide input for my class Fred?

FAQ: By adding a friend std::istream& operator>> (std::istream& i, Fred& fred);

FQA: This defective technique is symmetrical to the operator<< thing, which was discussed above. We'll use this opportunity to look at some advanced defects of this approach. For beginner's defects, go two FAQs back.

The complicated problems come from the same source as the simple problems: the idea that for each type, we want exactly one output function looking like a bit shift operator. Therefore, to create complicated problems, we'll need some complicated types. Luckily, C++ comes with one of the most complicated type systems in the solar system (itself an example of a simpler system).

Consider std::map. It's standard, and so is iostream. Does this mean that the operators reading and writing them are also standard? I think you know the answer. But no matter: you can create a template operator printing all kinds of map. So can I. We're both happy, unless we have to integrate our code into a single program some time later. We both used the same intuitive name - operator<< - for our printing functions. Normally we'd rename one of the two, but in this special case, this is really going to be ugly - for instance, other templates will break because they think everything can be printed with <<, and even if there aren't any, printMap(std::cout << "map: ", myMap) << std::endl is too ugly even for seasoned C++ developers, who do have pretty low standards.

The real fun starts when two people overload (or specialize or whatever they call it) the operators for special cases of types in conflicting ways. For instance, I know how to print std::map<std::string,T>, and you know how to print std::map<T,int>. The question is, who gets to print std::map<std::string,int>? The compiler is in trouble, and when a C++ compiler is in trouble, it immediately empties its bowl, dumping a nice, large, stinky error message right at our faces. Happy, happy, joy, joy, dear colleague.

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

FAQ: You can create a protected virtual method that each class will override to do the printing, and call it from a friend operator<<.

FQA: Fantastic, except for the protected and the friend operator<< part. Of course you can use a plain public virtual method instead.

[15.12] How can I open a stream in binary mode?

FAQ: Open the stream with the std::ios::binary flag. That way, the stream will not translate between '\n' and the target platform representation of line termination, which may be different (for instance, Windows uses CRLF).

FQA: With fopen, pass the "b" character in the options string. This whole issue is pretty annoying, especially if you work with binary files on a system where '\n' is not actually translated, and forget to open them as binary, and everything works, and then you port the program to a system where '\n' actually is translated, and then you have to find all those cases and open the files as binary. However, this is not the fault of C++. It is the fault of the distributed nature of the human race, which fails to standardize the simplest things.

Many programs may screw up your binary files due to this family of issues, for instance, many FTP clients will do so unless explicitly told otherwise.

[15.13] How can I "reopen" std::cin and std::cout in binary mode?

FAQ: There's no portable way to do it.

FQA: That's probably the fault of all those different incompatible systems rather than C++ or any other programming language.

[15.14] How can I write/read objects of my class to/from a data file?

FAQ: Read the section about serialization.

FQA: The FQA doesn't have a section about a serialization. Short summary: you're in for a rude awakening. There's no standard serialization mechanism in C++. Furthermore, there's no way to define a reasonable custom one since there's no reflection (no way to figure out the structure of an arbitrary object given a pointer to it, no way to create an object of a class given its name or some other sort of ID, etc.).

However, there are many custom packages for serialization (typically thousands of source code lines, requiring you to use hairy macros/templates for each serialized class member). Or you can roll your own, or you can dump the whole snapshot of your process to a file in non-portable ways (may be the cheapest thing to do more frequently than it sounds).

[15.15] How can I send objects of my class to another computer (e.g., via a socket, TCP/IP, FTP, email, a wireless link, etc.)?

FAQ: Read the section about serialization.

FQA: Keep your expectations low.

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

FAQ: Because \t expands to the tab character. Use \\t to say "backslash followed by t".

FQA: You have to escape certain things somehow, so this is perfectly legitimate - unlike the fact that the C++ standard library has no way to open a directory, for example.

[15.17] How can I tell {if a key, which key} was pressed before the user presses the ENTER key?

FAQ: The C++ standard doesn't even assume your system has a keyboard. So there's no portable way.

FQA: Good for the C++ standard, but is there a reason not to provide a standard interface for systems which do have a keyboard?

[15.18] How can I make it so keys pressed by users are not echoed on the screen?

FAQ: You can't. See above.

FQA: Yep, some systems don't have screens - we sure shouldn't support the ones that do.

Interestingly, the C++ standard assumes that your system has persistent files, and has separate output and error streams. Even more interestingly, it assumes that you might need Unicode output and locales. You'd be surprised to find out some of the strange places where devices carrying dead code supporting these features live.

[15.19] How can I move the cursor around on the screen?

FAQ: Using whichever way that works on your system. It's not related to the C++ standard.

FQA: You should have spotted a trend by now.

[15.20] How can I clear the screen? Is there something like clrscr()?

FAQ: There is, but not in the C++ standard. It's system-specific.

FQA: So are windows, sockets and regular expressions. You see, the C++ standard doesn't assume you have a screen, a network controller or hardware for regular expression matching.

The lame standard library is another one of those things making using C++ a joy.

[15.21] How can I change the colors on the screen?

FAQ: Depends on your system. The C++ standard doesn't refer to this.

FQA: But there's an ANSI standard that does. There are some ugly character sequences you can send to cout/stdout; you don't need to call any special functions. Many terminals support this.

Copyright © 2007-2009 Yossi Kreinin
revised 17 October 2009