Question about COUT

HelloWorld · #1 (**permalink**) 07-19-2007, 12:50 AM

Ok, I understand on how it works, but once again, I want to know what's really behind it! Here's the code that I used for the exercise from the Deitel book series (I love their book!)

PHP Code:

  #include "stdafx.h"
#include <iostream>
int main() {
int n1, n2, n3;
std::cout << "Input three integers: ";
std::cin >> n1;
std::cin >> n2;
std::cin >> n3;
std::cout << n1;
std::cout << n2;
std::cout << n3;
system("pause");
}

It seems like cout stores all of of the input value from the command line into arrays? (Is this true?)

Bench · #2 (**permalink**) 07-19-2007, 05:34 AM

I'm not sure how you came to that conclusion based on that program.

exactly how cin and cout works depends on which implementation of the STL you're using, and on the platform you're writing your program for - the C++ standard doesn't specify if or how any data is stored. before its transferred from the standard input device (which may have its own buffer - this is the case with keyboard input on Windows machines, but other platforms it may not be).

Just to clarify, the standard input device could be anything. On your computer, it might be the keyboard buffer - but it could be, for example, a mobile phone keypad, or a network connection, depending what sort of device you're programming for. Its up to the O/S (Which might be windows/linux, or may be firmware stored on a flash ROM chip) to handle the standard input device.

Understanding what goes on under the hood of cout and i/o streams isn't really very important - What is more important, with regards to streams, is that you can rely on stream objects to all behave in a consistant manner. That is to say, if you write a bit of code using cin/cout, you could replicate that same bit of code almost exactly with a stringstream (usually for string formatting or string parsing), or an fstream (file access).

The point is that the family of 'stream' libraries in C++ are designed to behave the same regardless of what input or output devices or buffers they may be attached to

rpgfan3233 · #3 (**permalink**) 07-19-2007, 07:49 AM

Like Bench, I'm not really sure how you arrived at the conclusion that you did. Could you explain what you mean by "input value" as well as what led you to that conclusion? I think I know what you mean by "input value" since you mention the command line, and you aren't working with the command line in terms of getting command line arguments and using them. However, it would be beneficial for you to explain a little bit more since it seems to be unclear for both Bench and me, which means that it could also be unclear for future readers that might have a similar idea.

HelloWorld · #4 (**permalink**) 07-19-2007, 09:42 AM

Quote:

I'm not sure how you came to that conclusion based on that program.

I came up with that conclusion based on the test that I made. As all of you may see that there's only one cout statement, however, there are many CIN, and somehow, my input that is seperated by spaces are automatically tokenized and stored to each of the variables by using CIN.

rpgfan3233 · #5 (**permalink**) 07-19-2007, 10:38 AM

If you are referring to something such as:

Code:

int n1 = 1, n2 = 2, n3 = 4; //unimaginative... I know :p
cout << "Enter three integers: ";
cin >> n1 >> n2 >> n3;

The answer is rather simple. What happens is it reads the value into a buffer and then tries to store that value in the first variable after it. I say "tries" because if you enter something like "1a", it will only store the 1 and then will set the badbit (or maybe failbit, I can't remember) because 'a' is invalid for int. This will prevent any other things from being read improperly (an advantage of C++ over C). Supposing that first value was a proper value, it would then try to read the next value. The idea is that with streams like cout and cin (basic_ostream& and basic_istream&, I believe is what they are actually), the order of operations is that the variable following cout/cin is outputted/inputted first. From there, it reads the next one, etc. The idea illustrated:

Code:

cout << "Hello World!" << endl;
(cout << "Hello World!") << endl; //same output as above
cout << ("Hello World!" << endl); //error because you are trying to treat the const char array/pointer with the value "Hello World!" as an output stream

Obviously, a character array that has a constant value/length defined cannot be written to as that would cause some severe memory issues. For example, endl is more than just a '\n' character. Yes, it often simply suits that purpose, but it also flushes the output buffer if the output stream is buffered (writes to the output stream whatever unwritten characters are left in the output buffer).

The same idea is true of the example with cin above:

Code:

cin >> n1 >> n2 >> n3;
((cin >> n1) >> n2) >> n3; //same as above
cin >> (n1 >> (n2 >> n3)); //error because you are trying to treat n2 as an input stream

HelloWorld · #6 (**permalink**) 07-20-2007, 12:19 AM

I also just found this out:

Quote:

When cin is used with the stream extraction operator, it reads characters until the first white-space character is reached.

from Deitel lol...

rpgfan3233 · #7 (**permalink**) 07-20-2007, 12:59 AM

That is true. Be aware that white-space characters include tabs, spaces and newlines. You don't necessarily need to use a space, in other words.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

The Following User Says Thank You to rpgfan3233 For This Useful Post:
HelloWorld (07-20-2007)