C++ for Unix quick-reference, with C variations

float myfloat = 3.7;
int   myint = 29;
char  mychar = '?';

mychar = char(myint);
myint = int(myfloat);

char to int conversion (and int to char) is done using the ascii tables for matching character values to integers

const int DaysInYear = 365;
const float Pi = 3.14;
const char Blank = ' ';

---

C Variation: const is not available, use #define instead,
e.g. #define DaysInYear 365


---

• provide type and name,
• if declared outside functions have global scope
• local variables declared as static retain their values across function calls
• variables which are declared in separately compiled file X must be declared as extern in all files except X

  int myint, anotherint, andanotherint;
  float fees = 0.0;
  static char initial = 'D';
  extern Boolean quit;
  

• Scope rules:

  • you cannot have two identifiers with the same scope and the same name

  • when scopes of like-named identifiers overlap the local scope is used

• The function layout is

  int sumandtruncatefloats(float f, float g)
  {
     return( int(f+g) );
  }
  

  The corresponding function declarations and calls might be

  int sumandtruncatefloats(float f, float g);
  ...
  float y = 2.914;
  int x;
  x = truncatefloat(3.7 + y);
  

• The return type is void if the function does not return a value

• By default, parameters are called pass-by-value, or value parameters (this means the function only gets a copy of the passed value)

  Pass-by-reference, or reference parameters, allow the function to alter the value of a variable passed to it

  void validswap(int &x, int &y)
  {
     int temp;
     temp = x;
     x = y;
     y = temp;
  }
  

  The call validswap(var1, var2); would swap the values of variables var1 and var2.

  Arrays and structs are automatically passed by reference - to prevent this prefix them with const in the parameter list:

  void foo(const char str[], int length);
  ...
  foo(mystring, SIZE);
  

  ---

  C Variation: const parameters unavailable. Pass by reference unavailable, must use pointers instead, e.g.

  void validswap(int *x, int *y)
  {
     int temp;
     temp = *x;
     *x = *y;
     *y = temp;
  }
  ...
  validswap(&var1, &var2)
  

  
  

  ---

• The available operators and precedence levels for evaluating expressions are as follows:
  1. ::
  2. . -> [] () x++ x--
  3. ++x --x ! *(deref) &(addr) +(unary) -(unary) new delete sizeof
  4. * / %
  5. + -
  6. << >> (i/o in C++, bitshifts in C)
  7. < <= > >=
  8. bitwise operators: ^ ~ & |
  9. == !=
  10. &&
  11. ||
  12. = += -= *= /= %=
  brackets are evaluated from the inside out, when dealing with multiple operators of the same level level evaluate from left to right

• Input and output require the inclusion of the iostream.h library

• Output of simple data types and strings can be carried out using cout

  int X = 0;       float y = 3.7;
  char c = 'a';    char myarray[4] = "xyz\0";
  
  cout << "X has value " << X << endl << "y has value " << y << endl;
  cout << "c has value " << c << endl << "myarray has value " << myarray << endl;
  

• Output of complex types (array elements, struct fields) must be done one part at a time

• setting the width of output to N characters can be done using setw(N), e.g.:

  cout << setw(3) << x;
  

• setting the precision of floating point output requires two functions, setiosflags and setprecision, e.g.:

  cout << setiosflags(ios::fixed) << setprecision(N) << x;
  

• setw, setprecision, and setiosflags all require the inclusion of library iomanip.h

• Input of simple data types and strings can be carried out using cin

  int X; float y; char c; char myarray[4];
  cin >> X >> y >> c >> myarray;
  

• cin does not take effect until the user hits enter (or return) and regards all fields as being separated by whitespace

• to read the next character, regardless of whether it is whitespace or not, use

  cin.get(c);  // where c is a char variable
  

• to read in an entire string of text, regardless of whether it contains whitespace of not, use

  cin.getline(mystring, Size-1);  
  // where mystring is an array of Size chars
  

  ---

  C Variation: Instead of cin, cout we have scanf, printf, with formatting strings. stdin, stdout, and stderr are the default i/o streams.

  long bignum;  int hexnum;  char c, word[20];
  scanf("%ld %s %x", &bignum, &word, &hexnum);
  printf("long: %ld, hex: %x, word: %s \n", bignum, hex, word);
  c = getchar();  /* equiv. to c = getc(stdin); */
  putchar(c);     /* equiv. to putc(c, stdout); */
  

  Available formats:
  • %d = int, %ld = long
  • %u = unsigned int, %lu = unsigned long
  • %o = octal, %x = hexadecimal, %lo and %lx for longs
  • %f for float or double, %e for exponential format
  • %c for char, %s for string
  
  

  ---

// C++ version                       
#include <iostream.h>            
#include <fstream.h>            

ofstream outfile;                 
ifstream infile;

void main()
{
   char infilename[SIZE];
   char outfilename[SIZE];
   char wholeline[SIZE];
   char c;

   cin >> infilename;
   cin >> outfilename;
   infile.open(infilename);
   if (infile.fail()) exit(1);
   outfile.open(outfilename);
   if (outfile.fail()) exit(2);
   infile.getline(wholeline, SIZE);
   while (!infile.eof()) {
      infile >> nextword;  
      infile.get(c);
      outfile << nextword;
      outfile << c;
   }
   infile.close();
   outfile.close();
}

---

C Variation:

#include <stdio.h>
#include <file.h>

FILE *infile, *outfile, *fopen();

void main()
{
   char infilename[SIZE], outfilename[SIZE];
   char c, wholeline[SIZE];

   infile = fopen("filename1", "r");
   outfile = fopen("filename2", "w");
   if (infile == NULL) exit(1);
   while (!feof(infile)) {
         fgets(wholeline, SIZE, infile);
         c = getc(infile);
         putc(outfile);
   }
   fclose(infile);
   fclose(outfile);
}




---

#include <stdlib.h>

void main(int argc, char *argv[])
{
    cout << "You called program " << argv[0] << " with arguments " << endl;
    for (int i = 1; i < argc; i++) {
        cout << argv[i] << " ";
    }
    cout << endl;
}

char commandline[256];
cin >> filename;
strcpy(commandline, "cat ");
strcat(commandline, filename);
strcat(commandline, " | wc >> ");
strcat(commandline, filename);
strcat(commandline, ".out");
system(commandline);

• the basic syntax for a complete if-else statement is

  if (<Boolean expression) {
     // statements if expression is true
  } else {
     // statements if expression is false
  }
  

  The "else" portion of the statement is optional.

• To test for a series of values of a variable you can use a structure like:

  if (x == 3) {
     // statements for x == 3
  } else if (x == 7) {
     // statements for x == 7
  } else if (x < 0) {
    // statements for x is negative
  } else {
    // statements for all other values of x
  }
  

• Similar results can be obtained with the switch statement, except that the <, >, <=, >= operators cannot be used directly (have to put them in the default code):

  switch (x) {
     case 3: // statements for x == 3
             break;
     case 7: // statements for x == 7
             break;
     default: if (x < 0) {
                 // statements for x is negative
              } else {
                 // statements for all other values
              }
  }
  

  Leaving out the break statements can allow us to group values for which we want similar behaviour, e.g.:

  switch (c) {
      case 'q':
      case 'Q': // statements for letter q
                break;
      case 'c':
      case 'C': // statements for letter c
                break;
      default:  // statements for all other letters
  }
  

• The syntax for a for loop is:

  for (<initialisation>; <continuance test>; <update> ) {
      <executable statements>
  }
  

  Example: consider all even integers from 0 through 98

  for (int x = 0; x < 100; x += 2) {
      // statements to be executed
  }
  

• The syntax for a while loop is:

  while (<Boolean expression) {
      // statements to be executed
  }
  

  Meaning: as long as the Boolean expression evaluates to true, carry out the body of the loop.

  Example: repeat until the user enters Q

  char c = 'C';
  while (c != 'Q') {
      // executable statements
      cout << "Enter Q to quit, C to continue" << endl;
      cin >> c;
  }
  

• The syntax for a do-while loop is:

  do {
     // statements to be executed
  } while (<Boolean expression>);
  

  Example: repeat until the user enters a positive value:

  do {
     cout << "Enter a postive integer" << endl;
     cin >> x;
  } while (x <= 0);
  

• Arrays are best declared using a const for the size of the array and a typedef for the array type, e.g.:

  // declare a type of array to hold 50 floats
  const int Size = 50;
  typedef float floatarray[Size];
  
  // declare an actual array variable 
  floatarray  myfloats;
  

• To use position i of the array like a variable of the appropriate type use arrayname[i], e.g.:

  myfloats[3] = 17.4;
  myfloats[2] = myfloats[3] + 4.1;
  x = sqrt(myfloats[2]);
  

• NOTE: myarray[i] is functionally identical to *(myarray + i)

• To pass arrays as reference parameters (function can modify them) follow this example:

  void fillarray(floatarray arr, int arrsize);
  
  void main()
  {
     floatarray myfloats;
     fillarray(myfloats, Size);
  }
  
  void fillarray(floatarray arr, int arrsize)
  {
     cout << "Enter " << arrsize << " floats" << endl;
     for (int i = 0; i < arrsize; i++) {
         cin >> arr[i];
     }
  }
  

  To pass arrays as value parameters prefix them with const in the parameter list, e.g. void viewarray(const floatarr arr, int arrsize);

• Multidimensional arrays can be thought of as 2-D or 3-D collections of storage locations rather than just 1-D, example of declaration:

  They are stored in row-major order

  const int Rows = 10;
  const int Columns = 16;
  typdef char grid[Rows][Columns];
  
  void main()
  {
     grid  Mygrid;
     for (int r = 0; r < Rows; r++) {
         for (int c = 0; c < Columns; c++) {
             Mygrid[r][c] = ' ';
         }
     }
  }
  

  To pass multidimensional arrays as parameters you must declare (in the parameter list) the size of all dimensions except the first, e.g.:

  int matrix[20][30];
  
  void initmatrix(int m[][30], int rows);
  

• Macież o rozmiarze nie ustalonym przez stałą predefiniowaną można stworzyć korzystając z dynamicznego przydziału pamięci tj. funkcji malloc/calloc i realloc (wówczas też działa odwoływanie się z []): int* intvector=(int *)malloc(100,sizeof(int))
• Żeby stworzyć tablicę dwuwymiarową wpierw stwórz wektor wskaźników do typu jaki ma być w tablicy, a następnie do każdego jego elementu wektor rządanego typu (wówczas dostęp jak do zwykłej tablicy: nnn[nx][ny])
• Korzystając z operatora new: int* intvector=new int[100]

• Strings are arrays of characters, can be input with cin, output with cout

• The string.h library contains commands to manipulate strings:
  • strcpy(string str1, string str2)
  • strncpy(string str1, string str2, int N)
  • int strlen(string str)
  • int strcmp(string str1, string str2)
  • int strncmp(string str1, string str2, int N)
  • int strchr(string str1, char c) - returns position of first occurance of c

• The recommended method for declaring a struct is a typedef, with syntax as follows:

  typedef struct {
      <type> field1;
      <type> field2;
      ...
      <type> fieldN;
  } <typename>;
  

  The syntax for accessing a field value is <varname>.<fieldname>

  Example: employee record

  typedef struct {
     string  surname;
     string  givenname;
     float   hourlywage;
  } EmployeeRec;
  
  void main()
  {
     EmployeeRec  employee1;
     strcpy(employee1.surname, "Wessels");
     strcpy(employee1.givenname, "Dave");
     employee1.hourlywage = 0.01;
  }
  

• You can have structs and arrays as fields within structs, and can create arrays of structs also

  typedef struct {
     string bossname;
     int numemployees;
     EmployeeRec staff[Size];
  } StaffList;
  
  void main()
  {
     StaffList Meagresoft;
  
     strcpy(Meagresoft.bossname, "Gill,Bates");
     int numemployees = Zillion;
     for (int slave = 0; slave < Size; slave++) {
         strcpy(Meagresoft.staff[slave].surname, " ");
         strcpy(Meagresoft.staff[slave].givenname, " ");
         Meagresoft.staff[slave].hourlywage = 35.0;
     }
  }
  

• Structs can be passed as (reference) parameters, just as with arrays:

  void initcomp(StaffList company);
  
  void main()
  {
     StaffList Meagresoft;
     initcomp(Meagresoft);
     ...
  }
  

• If X is a struct and Y is a field of X, then access Y via X.Y, but if X is a pointer to a struct then access Y via X->Y

enum booleflag { false, true };
booleflag = true;

• stdlib.h:
  • atoi(str), atof(str): convert strings to integers, doubles (return 0 if fails)
  • abs(i), labs(l): calculate absolute values for ints, longs

• math.h: may need to include -lm flag as last token in compilation command
  • fabs(d): absolute value for doubles
  • pow(d, d), exp(d): power, exponentiation
  • log(d), log10(d): natural log, log base 10
  • floor(d), ceil(d): rounding down and up
  • cos(d), sin(d), tan(d): cosine, sine, tangent

• ctype.h:
  • toupper(c), tolower(c): change case of char
  • isalphanum(c), isalpha(c), isdigit(c), ispunct(c), isspace(c), iscntrl(c), islower(c), isupper(c): boolean checks on type of char

#include <iostream.h>
#include <stdlib.h>
#include <time.h>

// C++ random number generator for values between X and Y

void main()
{
   int X, Y, result;
   srand((unsigned int)(time(NULL)));
   cin >> X >> Y;
   result = (rand() % (top + 1 - bottom)) + bottom;
   cout << result << endl;
}

#include <iostream.h>
#include <math.h>

// compile using g++ progname.cpp -o progname -fhandle-exceptions -lm

void main()
{
   float x, y;

   cout << "Enter a numerator and denominator" << endl;
   cin >> y >> x;

   // a try block contains exception handling
   // exceptions are invoked with throw()
   try {
      if (x == 0) throw("Divide by zero\0");
      if ((y/x) < 0) throw("Sqrt of negative\0");
      cout << "sqrt(" << y << "/" << x;
      cout << ") is " << sqrt(y / x) << endl;
   }
   // exceptions are handled with catch()
   catch(char str[]) {
     cout << str << endl;
     throw; // re-throws any unhandled errors
   }
}

---

C Variation: Not supported in C


---

int x;      // x is an integer
int *x_ptr; // x_ptr is a pointer to an integer

x_ptr = &x; // make x_ptr point to x
x = 3;
cout << x << " " << *x_ptr;  // print out 3 3

int foo(float x);

int (*f_ptr) ();   // f_ptr can point to any function
                   // which returns an int

f_ptr = foo;       // f_ptr points to function foo
y = (*f_ptr)(3.7); // equivalent to y = foo(3.7);

• need to know the size of the block of memory you wish to allocate, can be calculated using the sizeof function

  // want an array of 10 student records
  StudentRec *s_ptr;
  
  s_ptr = (StudentRec *)calloc(10, sizeof(StudentRec));
  // or alternatively
  // s_ptr = (StudentRec *)malloc(10 * sizeof(StudentRec));
  

• calloc czyści pamięć, a malloc nie
• żeby zmienić wielkość zarezerwowanej pamięci wykorzystaj realloc(s_ptr,20)
• zwolnienie zarezerwowanej pamięci - free(s_ptr)

---

Separate compilation/makefiles

The text below shows example header files and make files for separate compilation:
• there are four modules: mainprog, first, second, boolean
• boolean only has a header file (i.e. contains some relevant declarations, but no functions)
• mainprog, first, and second each have a program file (i.e. with a .cpp suffic, and containing the bodies of subroutines) and a header file (containing declarations)
• the routines in mainprog use information from each of the other files
• note the global variable myglobal is declared in first.cpp and used in main.cpp
• the routines in first and second need information from boolean
• boolean needs no information from other files
• using the makefile at the end, the user can type make mainprog, and the system will automatically compile exactly those files which have been altered since the last time make was run

       Dependencies:

          mainprog
        /     |    \
       /      |     \
     first.h  |      second.h
       \      |      /
        \     |     /
          boolean.h

//--------------------------------------------------------------------
// File boolean.h: gets included from several .h files 
//                 and the main program
//--------------------------------------------------------------------

#ifndef BOOLE

#undef TRUE
#undef FALSE

typedef int Boolean;

const int TRUE = 1;
const int FALSE = 0;

#define BOOLE
#endif

//--------------------------------------------------------------------
// File first.h: gets included from main, 
//               is header for first.cpp routines
//--------------------------------------------------------------------

#ifndef FIRST

#include "boolean.h"

int foo(char c);

#define FIRST
#endif

//--------------------------------------------------------------------
// File first.cpp: body of routines from first.h
//--------------------------------------------------------------------

#include 
#include "first.h"

int myglobal;
int foo(char c)
{
   cout << "C is " << c << endl;
   return( int(c) );
}

//--------------------------------------------------------------------
// File second.h: gets included from main, 
//                is header for second.cpp routines
//--------------------------------------------------------------------

#ifndef SECOND

#include "boolean.h"

char blah(int x);

#define SECOND
#endif

//--------------------------------------------------------------------
// File second.cpp: body of routines from second.h
//--------------------------------------------------------------------

#include 
#include "second.h"

char blah(int x)
{
   cout << "X is " << x << endl;
   return( char(x) );
}

//--------------------------------------------------------------------
// File mainprog.cpp: main program, uses seperately compiled files
//                    first.o and second.o
//--------------------------------------------------------------------

#include 
#include "first.h"
#include "second.h"
#include "boolean.h"

extern int myglobal;

void main()
{
   int mynum; 
   char mychar;

   mynum = foo('Z');
   mychar = blah(mynum);  
}


//--------------------------------------------------------------------
// File makefile: make mainprog - brings mainprog up to date,
//                                updates other files only if needed
//                make clean    - gets rid of .o files 
//--------------------------------------------------------------------

mainprog:	first.o second.o mainprog.cpp
	g++ first.o second.o mainprog.cpp -o mainprog

first.o:	first.cpp
	g++ -c first.cpp

second.o:	second.cpp
	g++ -c second.cpp

clean:
	rm -f *.o

---

C++ Classes

class Myclass {
public:
         // constructor
         Myclass();

         // destructor
         ~Myclass();

         // inline functions
         int returndata() { return(mydata); }
         void setdata(int x) { mydata = x; }

         // general functions
         void dosomework();

private:
         // private functions
         void adjustdata() { mydata++; }

         // private data
         int mydata;
         Myclass *classptr;
}

Myclass::Myclass()
{
  mydata = 0;
}

Myclass::~Myclass()
{
  delete classptr;
}

void Myclass::dosomework()
{
   classptr = new Myclass;
   classptr->setdata(mydata + 1);
}

void main()
{
   Myclass classvar;
   classvar->dosomework();
}

---

Threads in C

/*
 * compile using  cc -threads filename.c -o filename
 */

#include 
#include 
#include 

#define TRUE 1
#define FALSE 0

// macro for checking results of thread creation
#define CHECK(status, string) if (status == -1) perror(string)

// threads to be created
pthread_t  firstthread, secondthread;

// trigger to allow start of thread routines
int go = 0;

void ThreadRoutine(pthread_addr_t arg)
{
   // get thread identifier
   long int mynum;
   mynum = (long int) arg;

   // wait for release
   while ((!go)) {};

   // body of thread routine
   printf("Running thread routine %ld \n", mynum);
   system("ps -jlm");

   // terminate routine
   pthread_exit(arg);
}

void main()
{
   int status;

   // set thread identifier
   long int threadNum = 0;

   // value of exit from join call
   pthread_addr_t exit_value;
   
   // create the first thread and make sure its status is ok
   status = pthread_create( &firstthread, pthread_attr_default, 
                            (pthread_startroutine_t)ThreadRoutine, (pthread_addr_t) threadNum);
   CHECK(status, " :pthread_create bad status\n");

   // create the second thread and make sure its status is ok
   threadNum = 1;
   status = pthread_create( &secondthread, pthread_attr_default, 
                            (pthread_startroutine_t)ThreadRoutine, (pthread_addr_t) threadNum);
   CHECK(status, " :pthread_create bad status\n");

   // release the thread
   printf("Releasing the threads\n");
   go = 1;
   printf("Threads released\n");

   printf("Joining threads\n");
   status = pthread_join( firstthread, &exit_value);
   status = pthread_join( secondthread, &exit_value);
   printf("Detaching threads\n");
   status = pthread_detach( &firstthread );
   status = pthread_detach( &secondthread );
   printf("Threads terminated\n");
}

---

Semaphores in C

/*
 * compile using  cc -threads filename.c -o filename
 */

#include 
#include 
#include 

#define TRUE 1
#define FALSE 0

// number of threads to be created
#define Maxthreads 10

// macro for checking results of thread creation
#define CHECK(status, string) if (status == -1) perror(string)

// threads to be created
pthread_t  threads[10];

// mutual exclusion semaphore to be used
pthread_mutex_t MutexSem;

// trigger to allow start of thread routines
int go = 0;

void ThreadRoutine(pthread_addr_t arg)
{
   // get thread identifier
   long int mynum;
   mynum = (long int) arg;

   // wait for release
   while ((!go)) { };

   // body of thread routine
      
   // critical section
   pthread_mutex_lock(&MutexSem);  
   printf("Running thread routine %ld in critical section\n", mynum);
   pthread_mutex_unlock(&MutexSem);

   // terminate routine
   pthread_exit(arg);
}

void main()
{
   // return value from thread/semaphore function calls
   int status;

   // thread identifier
   long int threadNum;

   // value of exit from join call
   pthread_addr_t exit_value;
 
   // initialise the semaphore  
   status = pthread_mutex_init (&MutexSem, pthread_mutexattr_default);
   CHECK (status, " :Mutex_init bad status \n");

   // create the threads and make sure their status is ok
   for (threadNum = 0; threadNum < Maxthreads; threadNum++) {
      status = pthread_create( &threads[threadNum], pthread_attr_default, 
                               (pthread_startroutine_t)ThreadRoutine, 
                               (pthread_addr_t) threadNum);
      CHECK(status, " :pthread_create bad status\n");
   }

   // release the thread
   printf("Releasing the threads\n");
   go = 1;
   printf("Threads released\n");

   printf("Joining threads\n");
   for (threadNum = 0; threadNum < Maxthreads; threadNum++) 
        status = pthread_join( threads[threadNum], &exit_value);

   printf("Detaching threads\n");
   for (threadNum = 0; threadNum < Maxthreads; threadNum++) 
        status = pthread_detach( &threads[threadNum] );

   printf("Threads terminated\n");
}

---

Copyright © 1997, David M. Wessels. All rights reserved.