Mithat Konar
Derived from Differences Between C and C++ by Robert Niemann
2020-05-03

• <stdio.h> input/output
• <stdlib.h> standard utility functions
• <string.h> string operations
• <ctype.h> character class tests
• <math.h> math functions

• C's console and file I/O initially seems more complicated than C++.
• But it's sometimes preferable even in C++.

• printf() write a formatted string to the console
• scanf() read a formatted string from the keyboard
• putchar() write a single character to the console
• getchar() read a single character from the keyboard
• puts() write a string to the console
• gets() read a string from the keyboard

• Works with pointers to type FILE:

  FILE *fp;  /* create a pointer to FILE */

• fopen(), fclose() open/close a text file.
• feof() detect end-of-file marker in a file.
• fscanf() read formatted string from a file.
• fprintf() write formatted string to a file.
• fgets(), fputs() read/write a string.
• fgetc(), fputc() read/write a single character.

• You can't create classes or objects in C.
• You can't use any of C++'s predefined classes and objects.
• So, no
  • cout
  • cin
  • string
  • class
  • etc.

• boolean type
  • use int instead
• reference variables and function parameters
  • use pointers instead
• function overloading
  • use clever naming instead
• // single line comments (in older versions of C)
  • use /* multi-line syntax */ instead

• Use null-terminated character arrays.
• Use functions in string.h to manipulate strings.
  • C's string.h and C++'s cstring are essentially duplicates.

#include <string.h>
 
int main(void) {
  char s1[81] = {'H', 'e', 'l', 'l', 'o', '\0'};
  char s2[81] = "Hello World";
  char s3[] = "O hai.";
 
  int len1 = strlen(s1),  /*  5 */
      len2 = strlen(s2),  /* 11 */
      len3 = strlen(s3);  /*  6 */
 
  ...

• In older versions of C, you must define variables at the beginning of a function.
• Common practice today anyway.

int  main( )
{
    int a,b,c;
    float x,y,z;
    ...

• Function prototypes are not required.
• Best practice to use them anyway.

/* int foo(); <--optional */
 
int main()
{
    foo();
    return 0;
}
 
int foo()
{
    printf( "Hello world" );
}

• const is available only in newer versions of C.
• #define preprocessor directive typically used instead.

  #define PI 3.1415
  #define TAX_RATE 0.065

• #define also often used to create function-like macros.

  #define square(x) ((x) * (x))

• No static_cast<>, etc.
• Place the type you want a value cast to inside parenthesis before the value.

int myInt = 42;
double myDouble;
 
myDouble = (double)myInt/3;  /* cast myInt to a double */

• Use malloc and free.

• Allocating memory:

  int *x = (int *) malloc(sizeof(int));          /* a single int */
  int *xArry = (int *) malloc(sizeof(int) * 10); /* array of ints */

• Deallocating memory:

  free(x);
  free(xArry);

• No member functions.
• No access specifiers.
• You must use the struct keyword when defining struct variables.

  struct MyStruct
  {
      double x;
      double y;
  };
   
  struct MyStruct aStructInstance;  /* requires struct keyword */

• Nope. Nada. Epic fail. No classes or objects in C at all.

• C Reference Card (ANSI)
• A C Tutorial from Tutorials Point

• Look here.