Structure of C++ Program

A computer program is a sequence of instructions that tell the computer what to do.

Statements and expressions

The most common type of instruction in a program is the statement. A statement in C++ is the smallest independent unit in the language. In human language, it is analogous to a sentence. We write sentences in order to convey an idea. In C++, we write statements in order to convey to the compiler that we want to perform a task. Statements in C++ are terminated by a semicolon.

There are many different kinds of statements in C++. The following are some of the most common types of simple statements:

1 int x; 

2 x = 5; 

3 cout << x;

int x is a declaration statement. It tells the compiler that x is a variable. All variables in a program must be declared before they are used. We will talk more about variables shortly.

x = 5 is an assignment statement. It assigns a value (5) to a variable (x).

cout<< x; is an output statement. It outputs the value of x (which we set to 5 in the previous statement) to the screen.

The compiler is also capable of resolving expressions. An expression is an mathematical entity that evaluates to a value. For example, in math, the expression 2+3 evaluates to the value 5. Expressions can involve values (such as 2), variables (such as x), operators (such as +) and functions (which return an output value based on some input value). They can be singular (such as 2, or x), or compound (such as 2+3, 2+x, x+y, or (2+x)*(y-3)).

For example, the statement x = 2 + 3; is a valid assignment statement. The expression 2+3 evaluates to the value of 5. This value of 5 is then assigned to x.

Functions

In C++, statements are typically grouped into units called functions. A function is a collection of statements that executes sequentially. Every C++ program must contain a special function called main(). When the C++ program is run, execution starts with the first statement inside of main(). Functions are typically written to do a very specific job. For example, a function named Max() might contain statements that figures out which of two numbers is larger. A function named CalculateGrade() might calculate a student’s grade. We will talk more about functions later.

Libraries

Libraries are groups of functions that have been “packaged up” for reuse in many different programs. The core C++ language is actually very small and minimalistic — however, C++ comes with a bunch of libraries, known as the C++ standard libraries, that provide programmers with lots of extra functionality. For example, the iostream library contains functions for doing input and output. During the link stage of the compilation process, the libraries from the C++ standard library are the runtime support libraries that are linked into the program (this will be discussed further in lesson 1.4).

Taking a look at a sample program

Now that you have a brief understanding of what statements, functions, and libraries are, let’s look at a simple hello world program.

Consider our hello world program:

1.      view sourceprint?

2.      #include <iostream>

3.      int main( )

4.      { 

5.      using namespace std; 

6.      cout << "Hello world!"<<endl; 

7.      return 0; 

8.        }