a quick look at c for c++ programmers noah mendelsohn (with updates by mark sheldon) tufts...

A Quick Look at Cfor

C++ Programmers

Noah Mendelsohn (with updates by Mark Sheldon)Tufts University Email: [email protected]: http://www.cs.tufts.edu/~noah

COMP 40: Machine Structure and

Assembly Language Programming (Fall 2015)

mailto:[email protected]

© 2010 Noah Mendelsohn

Let’s look at some code


Hello world compared

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#include <iostream>#include <string>

using namespace std;

int main(int argc, char *argv[]){ string world = "world"; cout << "Hello " << world << endl;}

C++

#include <stdio.h>#include <stdlib.h>

int main(int argc, char *argv[]){

char *world = "world";

printf("Hello %s\n", world); return EXIT_SUCCESS; }

C



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C++ C

No namespaces in C



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intmain(int argc, char *argv[]){ string world = "world"; cout << "Hello " << world << endl;}

#include <stdio.h>

intmain(int argc, char *argv[]){

char world[] = "world";

printf("Hello %s\n", world); return 0; }

C++ C

C++: stream I/O w/coutC: stdio with stdout, printf, etc.



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C++ C

Format string allows substitution



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C++ C

Format strings:

%s – string%d – integer (decimal, signed)%u – integer (decimal, unsigned)%ld – integer (decimal, long)%x – integer (base 16 hex)%c – single ASCII character%5d – integer (5 chars wide)%-10s – string (10 chars left justified)Etc, etc.



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C++ C

\n = new line char\t = tab char\\ = \ charEtc.


Basic Datatypes


C and C++ mostly share basic data types

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char a single byte, capable of holdingone character in the local character set.

int an integer, typically reflecting thenatural size of integers on the host machine.

short int an integer, possibly smaller than int

long int an integer, possibly longer than intlong long int an integer, possibly longer than long

float single-precision floating point.double double-precision floating point.

Abbreviations: “short” is same as “short int”; “long” same as “long int”

Examples: int x; short int s; short s; double gpa;


Pointers

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char c; /* a single byte character */char *cp; /* a pointer to a single byte character */

A pointer variable holds a reference to some other variable.


What does this code do?

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int x; variable x holds an integerint y; variable y holds an integerint z; variable z holds an integer

int *ip; variable ip holds pointer to an integer

x = 2;y = 3;ip = &z;*ip = x + y;

printf(“First answer is %d\n”, z);

If you’re not sure, try this code yourself. Try changing it!


What does this code do?

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int x; variable x holds an integerint y; variable y holds an integerint z; variable z holds an integer

int *ip; variable ip holds pointer to an integer

x = 2;y = 3;ip = &z;*ip = x + y;

printf(“First answer is %d\n”, z);

*ip = *ip + z;printf(“Seconed answer is %d\n”, z);

If you’re not sure, try this code yourself. Try changing it!


Structured Data


Some structured data

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#include <stdio.h>

int main(int argc, char *argv[]){ struct student {

char *name; int age; };

struct student students[3] = { {"mary", 15}, {"bob", 9}, {"tina", 12},

};

unsigned int i;

(void)argc; (void)argv;

for (i=0; i < sizeof(students) / sizeof(struct student); i++) { printf("Student %s is %d years old.\n",

students[i].name, students[i].age);};

return 0; }



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struct student students[ ] = { {"mary", 15}, {"bob", 9}, {"tina", 12},

} ;

unsigned int i;


for (i = 0; i < sizeof(students) / sizeof(struct student); i++) { printf("Student %s is %d years old.\n",


return EXIT_SUCCESS; }

C has structs, not classes

structs have data only…no methods!



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} ;

unsigned int i;





Unlike C++: keyword struct required when naming a structured type



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} ;

unsigned int i;





Initializers more or less the same as C++



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} ;

unsigned int i;





We can leave out array bound if initializer determines the size –

same as C++


You saw printf and format strings earlier!

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} ;

unsigned int i;





You will want to learn about

printf and fprintf format specifications


Good Practice:Single Point of Truth


Single point of truth

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#include <stdio.h>intmain(int argc, char *argv[]){ struct student {


struct student students[ ] = { {"mary", 15}, {"bob", 9}, {"tina", 12},} ;

unsigned int i;




return 0; }

What’s going on here?



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#include <stdio.h>intmain(int argc, char *argv[]){ struct student {


struct student students[ ] = { {"mary", 15}, {"bob", 9}, {"tina", 12}, {“albert", 22},} ;

unsigned int i;




return 0; }

What if number of students changes?



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#include <stdio.h>#include <stdlib.h>int main(int argc, char *argv[]){ struct student {


struct student students[ ] = { {"mary", 15}, {"bob", 9}, {"tina", 12}, {“albert", 22},

} ;

unsigned int i;





There is a single point of truth for the number of students

Try to have single points of truth for anything in your program that’s likely to change, or on which multiple other things depend…if we change this structure, everything just works!


Why we use Hanson in COMP 40


What about abstract data types like list, or table?

Many modern languages have them built into the standard– Python, Ruby, etc., etc.

– C++ Standard Template Library (STL)

C does not standardize implementation of types like these

Hanson gives us C based ADT implementations that:– Are useful in our programs

– Teach us what’s going on in many of those higher level language implementations!

– Teach us many good techniques for building modular structures in C

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C vs C++ File I/O


Access to files and inputFeature C C++

Pre-opened streams stdin, stdout, stderr

cin, cout, cerr

Open a file FILE *fp fopen(filename, ”r”)

ifstream myfile(“filename”);

Typical use fprintf(stderr, ”error\n”);fprintf(fp,”Hi!”);

cerr << “error” << endl;myfile << “Hi!”;

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In both languages:

• The operating system pre-opens the three standard streams

• They can be redirected from the command line:

• myprog < somefile # stdin reads from somefile

• myprog > somefile # stdout writes to somefile

• otherprog | myprog # stdin of myprog is output of otherprog


More about C Strings


C characters

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(void)argc;(void)argv;

unsigned char var1 = 'N';unsigned char var2 = 'O';unsigned char var3 = 'A';unsigned char var4 = 'H';

/* %c prints as character %u prints unsigned integer */

printf("The number for %c is %u\n", var1, var1);printf("The number for %c is %u\n", var2, var2);printf("The number for %c is %u\n", var3, var3);printf("The number for %c is %u\n", var4, var4);

exit(EXIT_SUCCESS);}

Printing each one twice……in different formats!


C characters

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This program prints:

The number for N is 78The number for O is 79The number for A is 65The number for H is 72


C characters are integers!

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This program prints:

The number for N is 78The number for O is 79The number for A is 65The number for H is 72

Interesting…Characters are also numbers!



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C++: string type




C: arrays of characters





C++ C


Our first hello world

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#include <stdio.h>#inlcude <stdlib.h>




String literal gives array of characters


C arrays addressed by pointer to first element

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char *world = "world";char universe[] = “universe”;

printf("Hello %s\n", world);printf("Hello %s\n", universe);


These do almost the same thing

The relationship between arrays and pointers is subtle & important!

This one you need to research using K&R or Harbison & Steele


A trickier hello

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#include <stdio.h>#include <stdlib.h>#include <string.h>



printf("Hello %s\n", world);

world[1] = '\0';

printf("Hello %s your string is %d bytes long!\n", world, strlen(world));


What does this print?


If you understand this, you’re well on your way!

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char world[] = "world";printf("Hello %s\n", world);world[1] = '\0';printf("Hello %s your string is %d bytes long!\n",

world, strlen(world));world[3] = 'm';printf("Hello %s your string is %d bytes long!\n",

world, strlen(world));world[1] = 'o';world[4] = '\0';printf("Hello %s your string is %d bytes long!\n",

world, strlen(world)); return EXIT_SUCCESS; }

What does this print?


If you understand this, you’re well on your way!

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char *world = "world";printf("Hello %s\n", world);world[1] = '\0';printf("Hello %s your string is %d bytes long!\n",

world, strlen(world));world[3] = 'm';printf("Hello %s your string is %d bytes long!\n",

world, strlen(world));world[1] = 'o';world[4] = '\0';printf("Hello %s your string is %d bytes long!\n",

world, strlen(world)); return EXIT_SUCCESS; }

These examples show that:

1) The logical length of a C string is determined by the terminating null character ‘\0’2) Printing using %s and checking length with strlen() respect this3) In a correct program, there should be at least enough space for the string, but you may have allocated more4) In buggy programs, you fail to null terminate or index off the end of the allocated space


Memory allocation


There is no new in C! C++ new allocates and initializes objects:

– Car *myCar = new Car(V8, Blue); // Create a new car • The above allocates space and initializes all the data for mycar

– delete myCar;• Runs destructors and releases memory

– Also: int *ip = new int[len]; // allocate array delete[] ip; // delete array

– Also: std:vector // truly dynamic array

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There is no new in C! C++ new builds objects:

– Car *myCar = new Car(V8, Blue); // Create a new car • The above allocates space and initializes all the data for car

– delete myCar;• Runs destructors and releases memory

– Also: int *ip = new int[len]; // allocate array delete[] ip; // delete array

– Also: std:vector // truly dynamic array

C malloc/free allocate and free bytes:– struct car { ….members here… };struct car *car_p = malloc(sizeof struct car);• allocate unitialized bytes

– struct car *car_p = malloc(sizeof *carp);• Same, but keeps working if structure name changes: single point of truth!• You must check the return value to make sure it worked!

– free(car_p); /* frees the bytes */

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A Bit of History


C++ is an extension to C

C was invented many years earlier

C is a quite small language; C++ is much more complicated

C is much closer to the hardware, why we’re using it!– Good programmers know how each C construct compiles– No single C expression compiles to huge amounts of code

Most of C survives in C++– Primitive data types: int, float, double, etc– Control structures (if, while, function call)– Source structures (#include and preprocessor)– Much more

C does not have:– Classes, methods, namespaces, dynamic arrays, dynamic strings, IO

streams (in the C++ sense), inheritance, built in higher level ADTs (list, vector, deque, table, etc.)

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a quick look at c for c++ programmers noah mendelsohn (with updates by mark sheldon) tufts...

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