Using stdin & stdout in C & C++

Normally if I wanted to send some data to the screen (the console or stdout), I'd use the printf function. Here's an example.

printf("There are %d polar bears outside your window.", BearCount);

It works great. And if I wanted to send the same information to a file, I would use the fprintf function. It works the same way except you must specify a file to use. Such as:

// Declare pointer to the file.

FILE* pFileHandle;

// Saves the file in the EXE folder.

char Filename[] = "MyFile.txt";

// Open file for writing.

pFileHandle = fopen(Filename, "w");

// Write data to file.

fprintf(pFileHandle , "("There really are %d polar bears outside your window.\n\n", BearCount);

// Close the file.

int iReturn = 0;

iReturn = fclose(pFileHandle);

What if I wanted to send the data to the screen or a file or both? While the standard I/O library creates a type FILE for handling files, it also creates the objects stdin and stdout. I can use the same fprintf function in both cases and write code to change the value of pFileHandle.

For a file,

pFileHandle = fopen(…

but for the screen use:

pFileHandle = stdout;

fprintf(pFileHandle , "Print some data to a text file.\n\n");

OR

fprintf(stdout, "Print some data to stdout.\n\n");

The same principle applies to reading data from a file or from the keyboard (i.e., stdin). I can use the fgets function for either.

From a file with,

fgets(Text, num, pFileHandle);

or from the standard input:

fgets(Text, num, stdin);

Data to a maximum length of num is stored in the character array Text.

The reason for doing this should be obvious. You can easily switch between a file or standard I/O without repeating your code.
 

Test Code.

I tested the functions in Visual C++ 2010 as an console application.

// The standard library includes the system function.

#include <cstdlib>

// C++ standard I/O library.

#include <cstdio>

// Function prototypes.

int DataOut(FILE* pFile);

int DataIn(FILE* pFile, char* text);

// Max length of text input.

const int MAXLEN = 80;

int main()

{

     // Using stdin & stdout in C & C++

    

     // Header.

     printf("Using stdin & stdout instead of a File\n\n\n");

     // Output.

     printf("***** OUTPUT *****\n\n");

     // Declare pointer to the file.

     FILE* pFileHandle;

     // Saves the file in the EXE folder.

     char Filename[] = "MyFile.txt";

     // Open file.

     pFileHandle = fopen(Filename, "w");

     // Write data to file.

     DataOut(pFileHandle);

     // Close the file.

     int iReturn = 0;

     iReturn = fclose(pFileHandle);

     // Do the same but send to stdout.

     DataOut(stdout);

     printf("\n\n");

     // Input.

     printf("***** INPUT *****\n\n");

     // Declarations.

     char readline[MAXLEN];

     // Open file.

     pFileHandle = fopen(Filename, "r");

     // Get data.

     DataIn(pFileHandle, readline);

     // Close the file.

     iReturn = fclose(pFileHandle);

     printf("From the file: %s\n", readline);

 

     printf("Input with fgets: ");

     // Get data.

     pFileHandle = stdin;

     DataIn(pFileHandle, readline);

     // OR DataIn(stdin, readline);

     printf("Your input: %s\n", readline);

     // Keep console window open.

     system("pause");

     // Return some value.

     return 0;

} // end main

int DataOut(FILE* pFile)

{

     // Code to send data to file or screen.

     fprintf(pFile, "Print some data to a text file or to the screen.\n\n");

     return 0;

}

int DataIn(FILE* pFile, char* text)

{

     // Code to send data to file or screen.

     fgets(text, MAXLEN, pFile);

     return 0;

}

Output.