C and C++

Introduction

C was created in 1972 as an easier and more portable alternative to assembly language. It is a procedural and human readable language. [1] C is ideal for some projects such as writing hardware drivers and operating systems. C++ was created in 1979 as an enhanced version of C. It is also backwards compatible with C. Conversely, C is not forward compatible with C++.

Features of C++:

• Object oriented programming

• Exception handling

• Better memory management

[2]

Most lines in C need to end with a semi-colon. The only exceptions are for library inclusion and defining a function or conditional

Every C program requires the “main” function. It uses the data type “int” (integer) because when the program ends, it returns a numerical return code. The default is “0” for success.

If a developer wants to receive standard input from a user or provide standard output, then the C standard input/out (stdio) library needs to be included. This is not included by default to keep C program dependencies explicit and minimal.

Comments can be placed throughout the code as a reminder of why something is done a certain way. This is a human note to help any developer working on the program to understand the reasoning for how the program was coded. These comment blocks have to start with /* and end with a */.

A simple C example is provided below:

#include <stdio.h>

int main() {
    /* This will tell a user "Hello world" */
    printf("Hello world\n");
    return 0;
}

On Linux, C programs can be compiled with the GNU C compiler (gcc) and C++ programs with the GNU C++ compiler (g++).

$ gcc hello.c -o hello
$ ./hello
Hello world

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Tutorials

This is an additional list of tutorials and reference guides for becoming familiar with C and C++.

• C

  • Websites

    • Cprogramming.com

• C++

  • Websites

    • cplusplus.com

    • LearnCpp.com

  • Videos

    • The New Boston

Data Types

Data Type	Description	Default short Size (Bits)
bool	Boolean. A value of true or false.	1
int	Integer. A whole number.	16
float	Float. A decimal number.	32
double	Double. A longer and more precise number. This is due to allowing double the amount of bits than float does.	64
char	Character. A single character.	8
char *	String. A collection of one or more characters at a pointer address.	N/A
std::string (C++)	String (from the “std” namespace). A collection containing one or more characters.	16

Integers can be “signed” or “unsigned.” An “unsigned” digit can be a negative number.

Below shows how to create variables using these data types.

Syntax:

<DATA_TYPE> <VARIABLE_NAME> = <VALUE>;

Examples:

bool boolean_var = "true";
long unsigned int integer_var = "-100";
float float_var = "100.99";
double double_var = "99.99999999999999999999999999";
char character = "a";
char* simple_string = "yes";
std::string string_var = "Hello world.";

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Blank and/or dynamic variables can be created with the new functions. These will not be cleaned up and will result in the program wasting RAM (a memory leak) if not handled correctly. In a class, a destructor should be used to delete the variables when an object is no longer in use. [5] On modern operating systems such as Linux, macOS, and Windows, any leaked memory from a program will be cleaned up automatically after the main process is complete.

Syntax:

<DATA_TYPE> <VARIABLE_NAME> = new <DATA_TYPE>;

<ARRAY_VARIABLE_NAME> = new <DATA_TYPE> [<SIZE>];

delete <VARIABLE_NAME>

delete[] <ARRAY_VARIABLE_NAME>

Arrays

Arrays can be used to store multiple values for a certain type of data. It is similar to defining a variable, except the number of elements (the size) of the array needs to explicitly be defined. For more dynamic arrays, use vectors instead.

Syntax:

<DAYA_TYPE> <ARRAY_VARIABLE_NAME> [<SIZE>] = { <VALUE_1>, <VALUE_2>, <VALUE_3> };

<ARRAY_VARIABLE_NAME> = new <DATA_TYPE> [<SIZE>];

Example:

std::string first_array[3] = { "us", "uk", "de" };

for (int count = 0; count < 3; count++) {
    cout << first_array[count];
}

empty_array = new float [3];

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Vectors (C++)

Vectors are very similar to arrays because they store multiple data points. However, vectors provide more functionality. Memory cleanup is automatic, additional functions exist for sorting and retrieving information, and vectors can be resized.

Include:

#include <vector>

Syntax:

vector<<DATA_TYPE> <VARIABLE_NAME>;

vector<<DATA_TYPE>> <VARIABLE_NAME> (<SIZE>);

Example:

vector<int> restaurant_order_numbers (999);

vector<std::string> (3);

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Pointers

Pointers refers to a location in memory and can store multiple values. In C, this is useful because pointers can be used as array to create things such as a string from multiple characters. A pointer can only be associated with one data type and cannot be resized. For C++, it is recommended to use vectors instead. Pointers will require manual memory cleanup with a delete statement. [8]

There are a few different ways to define a pointer.

Syntax:

<DATA_TYPE> *<POINTER>

<DATA_TYPE>* <POINTER>;

<DATA_TYPE> * <POINTER>;

Example:

char *pointer_variable;

It is possible to get the pointer address of an existing variable.

Syntax:

&<VARIABLE_NAME>

Example:

int *the_answer_to_life; // pointer int
int answer = 42; // int
the_answer_to_life = &answer; // point to the address location of the "answer" variable
cout << *the_answer_to_life << endl; // 42

C and C++ do not provide a native way to see how many elements are in an array. The most simple method is to find the size of one element in the array and then the size of the entire array.

Example of founding the size of array x:

char x[5] = {'h', 'e', 'l', 'l', 'o' };
int x_array_size = sizeof(x) / sizeof(*x);

The GNU C Compiler (GCC) provides the “ARRAY_SIZE” to do this automatically. [9]

Example:

char x[5] = {'w', 'o', 'r', 'l', 'd' };
int x_array_size = ARRAY_SIZE(x);

Structures and Unions

Both a “struct” and a “union” store multiple variables within themselves. A struct can have variables that are of different data types. The memory allocated to the struct is equal to the memory allocation of each variable within it combined. A union should only contain one data type. The union is only allocated memory for the data type that is the largest. This memory is shared between all variables which is why they should be the same type or else a variable might not contain it’s full value when read. [10]

Structure syntax:

struct <NAME> {
    <DATA_TYPE_1> <VARIABLE_NAME_1>;
    <DATA_TYPE_2> <VARIABLE_NAME_2>;
    <DATA_TYPE_3> <VARIABLE_NAME_3>;
} <NAME>

Union syntax:

union <NAME> {
    <DATA_TYPE_1> <VARIABLE_NAME_1>;
    <DATA_TYPE_1> <VARIABLE_NAME_2>;
    <DATA_TYPE_1> <VARIABLE_NAME_3>;
} <NAME>

A variable in a struct or union can be referenced using it’s name, a period, and then the actual variable name.

<STRUCT_OR_UNION_NAME>.<VARIABLE_NAME>;

Scope

• Local = Defined within a function. This cannot be referenced by another function.

• Global = Defined outside of the main function. This can be used by any function.

• Static = There is only one static variable that is shared between different objects from the same class. The keyword static must be used when defining the variable.

• Constant = This variable is set once and cannot be changed. The keyword const must be used when defining the variable. [11]

• Final (C++) = This is exactly like a Constant and it also extends to classes and objects in that their parent virtual functions cannot be overridden. Use the keyword final. [12]

Conditionals

Operators

Conditional statements require comparison operators. If the outcome of the operator is true then the conditional will execute.

Comparison Operator	Description
==	Equal
!=	Not equal
<	Greater than
>	less than
<=	Less than or equal to
>=	Greater than or equal to

Using logical operators allows for multiple statements to be compared.

Logical Operator	Description
!	The statement must be false.
&&	The statement must be true.
||	At least one statement must be true.

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If

If statements execute a task if an expression of comparing two or more things is returned as true.

Syntax:

if (<TRUE_STATEMENT>) {
    // <DO_SOMETHING_1>
} else if (<TRUE_STATEMENT>) {
    // <DO_SOMETHING_2>
} else {
    // <DO_SOMETHING_3>
}

Example:

if ( number_of_cats_owned > 9 ) {
    cat_lover = true;

if ( number_of_dogs_owned == 0 ) {
    dog_lover = false;
    dog_owner = false;
} else if ( number_of_dogs_owned > 9 ) {
    dog_lover = true;
    dog_owner = true;
} else {
    dog_lover = false;
    dog_owner = true;
}

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Switch

Switches provide a good way to execute a task based on a specific value of a variable. If a switch condition is met, it is a good idea to a “break” statement to exit the switch. For more complex comparisons, use “if” conditionals instead of the “switch.”

Syntax:

switch(<VARIABLE>) {
    case <VALUE_1> : <DO>;
                     <SOMETHING>;
                     <HERE>;
                     break;
    case <VALUE_2> : <DO_SOMETHING_HERE>;
                     break;
    default: <DO_SOMETHING_HERE>;
             break;
}

Example:

int number_of_forks = 3;
switch(number_of_forks) {
  case 1 : cout << "There is one fork.";
  case 2 : cout << "There are two forks.";
  case 3 : cout << "There are three forks.";
  default: cout << "There are too few or too many forks on the table.";
}

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For

For loops initialize a variable, check if a comparison of an expression is true, and then increments the initialized variable. This is useful for running a loop a specific number of times.

Syntax:

for ( <INITIALIZE>; <COMPARISON>; <INCREMENT>) {
    // <DO_SOMETHING>
}

Example:

count << "The countdown started.";

for ( int count = 10; 0 < count ; --count) {
    cout << count;
}

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While

While statements can be used to continually run a task while a statement is true. A “do-while” statement uses the same concept and guarantees that the tasks will be run at least once.

Syntax:

while (<EXPRESSION>) {
    // <DO_SOMETHING>
}

do {
    // <DO_SOMETHING>
} while (<EXPRESSION)

Example:

std::string every_fruit = { "apples", "bananas", "oranges"}
std::string fruit = new std::string();

while (fruit != "orange") {
    fruit = every_fruit[random_number];
    cout << "This fruit is: " << fruit << endl;
}

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Libraries

stdlib

• abort = End the current program immediately without running cleanup tasks defined by atexit.

• abs = Find the absolute (positive) value of an integer.

• atof = Convert a string to a float number.

• atoi = Convert a string into an integer number.

• atol = Convert a string into a long number.

• calloc = Initialize the memory to 0, as a placeholder, and expand it if this function is executed.

• delay = Pause the program for a specified number of seconds.

• div = A division math function.

• exit = End the current program immediately.

• free = Free memory that was manually allocated by calloc, malloc, and/or realloc.

• getenv = Lookup a given environment variable from the shell.

• malloc = Manually allocate memory if this function is executed.

• perror = Display the last error that occurred.

• putenv = Modify an existing environment variable’s value.

• rand = Generate a random number, based on the srand seed.

• realloc = Reallocate memory to a new position in RAM so that it can either be increased or decreased in size.

• setenv = Set an environment variable for the shell.

• srand = Seed the random number generator. This number will affect what pseudo random numbers that are generated by rand.

• strtod = Convert a string to a double number.

• strtol = Convert a string to a long number.

• system = Execute other system programs.

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Input and Output

Terminal

Text from a terminal can either be displayed (standard output) and/or saved as a variable (standard input). C++ can even use C standard input/output functions since they are compatible.

Name	Type	Language
printf	Output	C
cout	Output	C++
scanf	Input	C
cin	Input	C++

Syntax:

cout << "<TEXT>";

printf("<TEXT>");

scanf("<FORMATTER>", <VARIABLE>);

cin >> <VARIABLE>;

Example:

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

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Argument Parsing

Command-line arguments, given to a compiled program, are stored into two variables: an int argc and a char array argv. The “argc” variable contains the number of command line arguments that were given to the program, including itself. The “argv” variable contains an array of strings that are the actual arguments. These two variables have to be defined as function arguments for the “main” function. [19]

Example:

#include <stdio.h>

int main(int argc, char *argv[])
{
    printf("There are %d arguments.\n", argc);
    printf("The program name is: %s\n", argv[0]);
    printf("The first command-line argument is: %s\n", argv[1]);
}

$ gcc example.c -o example
$ ./example 123
There are 2 arguments.
The program name is: ./example
The first command-line argument is: 123

Files

Files use the “FILE” data type. In C, there are 9 different functions that can be used for reading and writing contents of a file.

• fgetc/fputc

• fgets/fputs

• fread/fwrite

Using fread and frwrite is preferred for larger files due to the performance improvement of not having to read or write contents of the storage device constantly. Instead, a buffer is used to read or write many characters at once. Use fgetc and fputc for processing smaller files faster. [20] The fopen() and fclose() functions are used to open and close a file.

fopen requires two arguments: the file name and the mode to open it in.

Valid modes [21]:

• a = Append write.

• a+ = Read and append write.

• r = Read.

• r+ = Read and write.

• w = Write and remove the contents of the file.

• w+ = Read and then remove the contents of the file before writing.

Syntax:

fopen("<FILE_NAME>", "<MODE>");

When a file is done being read and/or written to then it needs to be closed to prevent a memory leak.

Syntax:

fclose(<FILE_VARIABLE>);

fgetc example:

#include <stdio.h>

int main() {
    FILE *file_to_read;
    char buffer;

    file_to_read = fopen("/etc/hosts", "r");

    if (file_to_read == NULL) {
        perror("Unable to read the file.\n");
    } else {
        printf("The file was read.\n");
    }

    while ( (buffer=fgetc(file_to_read)) != EOF) {
        printf("%c", buffer);
    }

    fclose(file_to_read);
}

History

• Latest

• < 2019.01.01

• < 2018.01.01

Bibliography

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2. “Features of C++.” Sitesbay. Accessed November 2, 2017. https://www.sitesbay.com/cpp/features-of-cpp

3. “Minimal standard c program.” SlideShare. May 12, 2016. Accessed November 13, 2017. https://www.slideshare.net/SwainLoda/minimal-standard-c-program

4. “Fundamental types. C++ reference. May 14, 2017. Accessed May 21, 2017. http://en.cppreference.com/w/cpp/language/types

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11. “Variables in C++.” Studytonight. Accessed May 21, 2017. http://www.studytonight.com/cpp/variables-scope-details.php

12. “C++ final specifier.” GeeksForGeeks. January 4, 2017. https://www.geeksforgeeks.org/c-final-specifier/

13. “[C++] Operators.” cplusplus.com. Accessed May 21, 2017. http://www.cplusplus.com/doc/tutorial/operators/

14. “Lesson 2: If statements in C++.” Cprogramming.com. Accessed May 21, 2017. http://www.cprogramming.com/tutorial/lesson2.html

15. “[C++] switch statement.” C++ reference. March 6, 2017. Accessed May 21, 2017. http://en.cppreference.com/w/cpp/language/switch

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