C++ Basic Component

Comments

Comments should be used to enhance the readability of your code.

• Giving a high-level overview of any function you write whose purposes might not be immediately clear from its name.
  • Parameters
  • Output
  • Return Value
• Give high-level comments that tell what chunks of code do

Block Comment

/* This is a comment!  
 * This is also part of the same comment!  
 * Qt Creator adds a star on each line for formatting purposes when we use this  
 * type of comment.  
Even though we don't have a star at the beginning of this line, it's still part of  
our comment!  
 * When we hit the end of this line, the star-slash causes our comment to end. */

Single Line Comment

// Comment 1
// Comment 2, No spanning lines

#include Directives

The include directives import libraries of pre-written code.

Syntax: including standard system libraries:

#include < LIBRARY_NAME >

including user-defined libraries:

#include " LIBRARY_NAME "

The statements begin with hash # are preprocessor command or directives. This usually includes #include and #define statements, which don't need semicolons ; to terminate the end.

Namespaces

A namespace is a named collection of functions (and other entities) that are grouped together. Functions called must have their namespaces given.

This resolves the problem of different library possessing the functions of the same name. Just seperate them by their namespace.

net::createConnection();  
db::createConnection();

Standard C++ libraries have the namespace std.

#include <iostream>

int main() {
   std::cout << "Hello, world!" << std::endl;
   return 0;
}

To skip specifying namespace for every function, use the using namespace statement.

#include <iostream>
using namespace std;

int main() {
   // removed both occurrences of std:: below
   cout << "Hello, world!" << endl;
   return 0;
}

The main() Function

Functions in C++ requires us to specify the type of parameters and the return type.

Syntax for defining a function:

DATA_TYPE FUNCTION_NAME (PARAMETERS) {
    STATEMENT(S);
}

• Code blocks seperated by curly braces define the scope of the function.
• White spaces are mostly ignored by compiler but good for human readable code.
• Indentation for readibility.

Special for main() Function

The C++ compiler goes line-by-line through our code, starting on the first line and working its way down.

Program execution always automatically begins in main(). A program that doesn't have a main() function won't compile.

int main() {
  statement;
  statement;
  ...
  statement;
  return 0;
}

The main function is a special function that inddicates the overall starting point of the program

• main can call other functions
• returns an integer exit code to the OS indicating exit status (0 if no error else fatal error cases) as a signal for other programs

Return Statements

Syntax for returning a value from a function:

return EXPRESSION;

The EXPRESSION can be any expression that evaluates to the appropriate return type for our function.

The cout Stream

cout is what we call a stream. By default, it acts as a direct line to our terminal. Anything we send to it with the << operator gets printed to the screen.

The endl String Manipulator

endl causes a newline character to print to the screen.

#include <iostream>
using namespace std;

int main() {
   cout << "Hello, world!"; // Withou endl
   cout << "CS106B is awesome, and so are you!" << endl;
   cout << "switch line" << endl;
   return 0;
}

Constants

Constants' value cannot be changed during the process of execution. A constant must be initialized when it is defined and once initialized, it cannot be assigned a new value or changed in any way.

Syntax for defining a constant:

const TYPE NAME = VALUE;

• Naming conventions: use upper case letters for constants

Variables and Data Types

Some simple data types. See more and detailed on [[SJTU/SJTU-CS1601/Data Types]]. C++ requires us to formally declare every variable we want to use and give it a specific data type that cannot be changed after declaration.

data type keyword	type description	example(s)
int	integer (whole number)	-3, 0, 1, 1051
float	floating point number (real number)	3.14159
double	floating point number (real number)	3.14159
char	character (single glyph)	'q', 'P', '$' (requires single quotes)
string	string of characters	"Hello, world!" (requires double quotes)
bool	Boolean	true, false (no quotes)

A variable declaration is a statement wherein we bring a new variable into existence and give it a data type. Declaration gives the following properties of a variable:

• Name
• Type
• Lifetime
• Scope

The syntax for that in C++ is:

DATA_TYPE VARIABLE_NAME;

Common practice for a variable:

• Definition (without initial value)
• Assignment (give it a new value)
• Initialization (with a initial value)

Syntax for initialization:

DATA_TYPE VARIABLE_NAME = VALUE;
DATA_TYPE VARIABLE_NAME(VALUE);

Identifiers

The names used for variables, functions, types, constants, etc are known as identifiers. Rules for identifier formation:

• The name must start with [a-zA-Z_]
• The rest can contain only [a-zA-Z0-9_]
• Not in the C++ keywords

Common Pitfall with Variables

• Undeclared variable
• Type mismatch
• Attempting to change a variable's data type
• Redefinition of variables

Uninitialized Variables

In C++, variables are not automatically initialized. By default, if you don't assign a value to a variable, it contains garbage.

While Loops

Syntax:

while (CONDITION) {
  STATEMENT(S)
}

Example:

#include <iostream>
using namespace std;

int main() {
   int i = 1;

   while (i < 5) {
      cout << i << endl;
      i++;
   }

   return 0;
}

The ++ Operator

See more on [[Increment Decrement Operators]]

For Loops

Syntax:

for (INITIALIZATION_STATEMENT); CONDITION; POST_ITERATION_STATEMENT) {
  STATEMENT(S)
}

• Initialize statement: executed once
• Condition: evaluated before kicking off an iteration
• Post-incrementation statement: executed at the end of each iteration

Example:

#include <iostream>
using namespace std;

int main() {
   for (int i = 1; i < 5; i++) {
      cout << i << endl;
   }

   return 0;
}

For-Each Loops (Range-Based Loops)

Iterate through all of the elements in a container.

Syntax:

for (DATA_TYPE VARIABLE_NAME : CONTAINER) {
    STATEMENT(S) 
}

Example:

#include <iostream>
using namespace std;

int main() {
   string s = "giraffe";

   // Initially, ch is set equal to the first character in s. With each subsequent
   // iteration of the loop, ch moves forward by one character in the string.
   for (char ch : s) {
      cout << ch << endl;
   }

   return 0;
}

Conditional (If-Else) Statements

Syntax:

if (CONDITION) {
  STATEMENT(S)
} else if (CONDITION) {
  STATEMENT(S)
} else {
  STATEMENT(S)
}

• One if
• Zero or more else if
• Zero or one else

Comparision operators

operator	meaning
<	less than
<=	less than or equal to
>	greater than
>=	greater than or equal to
==	equal to
!=	not equal to

Boolean Opeators

operator	meaning
!	Boolean "NOT"
&&	Boolean "AND"
||	Boolean "OR"

NOT

A compound expression needs to be warped up by prentheses in order to be negated by !. This is because of [[Operator Precedence]].

AND and OR

Binary operators in C++ are short circuit operators.

<left> && <right>;
<left> || <right>;

• False values in C++: 0, false
• True values in C++: everything else

&&

1. Evaluate the subexpr <left>
2. If the result is a false value v, then the expression evaluates to v
3. Otherwise, the expression evaluates to <right>

||

1. Evaluate <left>
2. If the result is a false value v, then the expression evaluates to v
3. Otherwise, the expression evaluates to <right>

Common Pitfalls

The following always evaluates to true:

(numCupcakes == 1 || 2)

Use || twice

(numCupcakes == 1 || numCupcakes == 2)