C++ Programmers for A Quick Look at C COMP 40: Machine ...+.pdfC++ C Format specifier allows...

A Quick Look at Cfor

C++ Programmers

Richard Townsend (Original slides by Noah Mendelsohn, with updates by Mark Sheldon)

Tufts University Email: [email protected]

COMP 40: Machine Structure and

Assembly Language Programming (Fall 2019)

© 2010 Noah Mendelsohn

Let’s look at some code


Hello world compared

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#include #include

using namespace std;

int main(){ string world = "world"; cout



4

#include #include





5

#include #include





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#include

int main(){

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

}

#include #include





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#include

int main(){


}

#include #include





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#include

int main(){


}

#include #include




#include

int main(){


}


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#include #include




The string in Hello World

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#include

int main(){


}

C: arrays of charactersHuh?How come this is a pointer?

In many cases, a reference to a C array turns into a pointer to the first element……using pointers to the beginning of an array is very common in C. Read about this in Kernighan & Ritchie, Section 5.3 (link provided on the class website)!


The string in Hello World

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#include

int main(){

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 (section 5.5).


Basic Datatypes


C and C++ mostly share basic data types

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char a single byte, capable of holding one 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;

To print size of a type in bytes: printf(“%ul\n”, sizeof(float));


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);

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


More about char and C Strings


C characters

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#include #include

int main(){

char var1 = 'N';char var2 = 'O';char var3 = 'A';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);

}

Printing each one twice……in different formats!


C characters

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#include #include

int main(){




}

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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#include #include

int main(){




}

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!


A trickier hello

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#include #include #include

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

char world[] = "world";

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

world[1] = '\0';

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

}


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 %lu bytes long!\n",

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

world, strlen(world));world[1] = 'o';world[4] = '\0';printf("Hello %s your string is %lu 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)); }

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 this.3) In a correct program, there should be at least enough

space for the string, but you may have allocated more.4) In buggy programs, you fail to null terminate or index off

the end of the allocated space.


Structured Data


#include #include

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

char *name; int age; };

struct student students[ ] = { {"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); }

}

“Unused Arguments” compiler warnings

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Suppress compiler warning about unused arguments

--same as C++


Some structured data

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#include #include




} ;

unsigned int i;




}

C has structs, not classes

structs have data only…no methods!


#include #include




} ;

unsigned int i;




}


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


#include #include




} ;

unsigned int i;




}


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Initializers more or less the same as C++


#include #include




} ;

unsigned int i;




}


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

same as C++


#include #include




} ;

unsigned int i;




}

You saw printf and format strings earlier!

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You will want to learn about

printf and fprintf format specifications


Good Practice:Single Point of Truth


#include #include




} ;

unsigned int i;




}

What’s going on here?

Single point of truth

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#include #include



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

} ;

unsigned int i;




}


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What if number of students changes?


#include #include



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

} ;

unsigned int i;




}


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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;fp = fopen(filename, ”r”);

ifstream myfile;myfile.open(“filename”);

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

cerr


Memory allocation


There is no new in C!▪ C++ 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++ 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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Rule of thumb: The first statement after a call to

handles

▪ 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 *car_p);• 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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