Chapter introduction
Welcome to the first primary chapter of these C++ tutorials!
In this chapter, we’ll take a first look at a number of topics that are essential to every C++ program. Because there are quite a few topics to cover, we’ll cover most at a fairly shallow level (just enough to get by). The goal of this chapter is to help you understand how basic C++ programs are constructed. By the end of the chapter, you will be able to write your own simple programs.
In future chapters, we’ll revisit the majority of these topics and explore them in more detail. We’ll also introduce new concepts that build on top of these.
In order to keep the lesson lengths manageable, topics may be split over several subsequent lessons. If you feel like some important concept isn’t covered in a lesson, or you have a question that isn’t answered in the current lesson, it’s possible that it’s covered in the next lesson.
Statements
A computer program is a sequence of instructions that tell the computer what to do. A statement is a type of instruction that causes the program to perform some action.
Statements are by far the most common type of instruction in a C++ program. This is because they are the smallest independent unit of computation in the C++ language. In that regard, they act much like sentences do in natural language. When we want to convey an idea to another person, we typically write or speak in sentences (not in random words or syllables). In C++, when we want to have our program do something, we typically write statements.
Most (but not all) statements in C++ end in a semicolon. If you see a line that ends in a semicolon, it’s probably a statement.
In a high-level language such as C++, a single statement may compile into many machine language instructions.
For advanced readers
There are many different kinds of statements in C++:
- Declaration statements
- Jump statements
- Expression statements
- Compound statements
- Selection statements (conditionals)
- Iteration statements (loops)
- Try blocks
By the time you’re through with this tutorial series, you’ll understand what all of these are!
Functions and the main function
In C++, statements are typically grouped into units called functions. A function is a collection of statements that get executed sequentially (in order, from top to bottom). As you learn to write your own programs, you’ll be able to create your own functions and mix and match statements in any way you please (we’ll show how in a future lesson).
Rule
Every C++ program must have a special function named main (all lower case letters).
When the program is run, the statements inside of main are executed in sequential order.
Programs typically terminate (finish running) after the last statement inside function main has been executed (though programs may abort early in some circumstances, or do some cleanup afterwards).
Functions are typically written to do a specific job or perform some useful action. 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 from a set of test scores. A function named printEmployee might print an employee’s information to the console. We will talk a lot more about functions soon, as they are the most commonly used organizing tool in a program.
Nomenclature
When discussing functions, it’s fairly common shorthand to append a pair of parenthesis to the end of the function’s name. For example, if you see the term main() or doSomething(), this is shorthand for functions named main or doSomething respectively. This helps differentiate functions from other things with names (such as variables) without having to write the word “function” each time.
In programming, the name of a function (or object, type, template, etc…) is called its identifier.
Dissecting Hello world!
Now that you have a brief understanding of what statements and functions are, let’s return to our “Hello world” program and take a high-level look at what each line does in more detail.
#include <iostream>
int main()
{
std::cout << "Hello world!";
return 0;
}Line 1 is a special type of line called a preprocessor directive. This #include preprocessor directive indicates that we would like to use the contents of the iostream library, which is the part of the C++ standard library that allows us to read and write text from/to the console. We need this line in order to use std::cout on line 5. Excluding this line would result in a compile error on line 5, as the compiler wouldn’t otherwise know what std::cout is.
Line 2 is blank, and is ignored by the compiler. This line exists only to help make the program more readable to humans (by separating the #include preprocessor directive and the subsequent parts of the program).
Line 3 tells the compiler that we’re going to write (define) a function whose name (identifier) is main. As you learned above, every C++ program must have a main function or it will fail to link. This function will produce a value whose type is int (an integer).
Lines 4 and 7 tell the compiler which lines are part of the main function. Everything between the opening curly brace on line 4 and the closing curly brace on line 7 is considered part of the main function. This is called the function body.
Line 5 is the first statement within function main, and is the first statement that will execute when we run our program. std::cout (which stands for “character output”) and the << operator allow us to display information on the console. In this case, we’re displaying the text “Hello world!”. This statement creates the visible output of the program.
Line 6 is a return statement. When an executable program finishes running, the program sends a value back to the operating system in order to indicate whether it ran successfully or not. This particular return statement returns the integer value 0 to the operating system, which means “everything went okay!”. This is the last statement in the program that executes.
All of the programs we write will follow this general template, or a variation on it.
Author’s note
If parts (or all) of the above explanation are confusing, that’s to be expected at this point. This was just to provide a quick overview. Subsequent lessons will dig into all of the above topics, with plenty of additional explanation and examples.
You can compile and run this program yourself, and you will see that it outputs the following to the console:
Hello world!
If you run into issues compiling or executing this program, check out lesson 0.8 -- A few common C++ problems.
Syntax and syntax errors
In the English language, sentences are constructed according to specific grammatical rules that you probably learned in English class in school. For example, in writing, normal sentences end in a period. The set of rules that describe how specific words (and punctuation) can be arranged to form valid sentences in a language is called syntax. For example, “My house painted is blue” is a syntax error, because the ordering of the words is unconventional. “All your base are belong to us!” is another notable one.
The C++ programming language also has a syntax, which describes how the elements of your program must be written and arranged in order for the program to be considered valid. When you compile your program, the compiler is responsible for making sure your program follows these syntactical rules. If your program does something that deviates from the syntax of the language, the compiler will halt compilation and issue a _syntax error.
Unlike the English language, which allows for a lot of ambiguity, the syntax rules of C++ are strictly defined and upheld. Syntax errors are common. Fortunately, such errors are typically straightforward to find and fix, as the compiler will generally point you right at them. Compilation of a program will only complete once all syntax errors are resolved.
Since the syntax for most statements requires those statements to end in a semicolon, let’s see what happens if we omit the semicolon on line 5 of the “Hello world” program, like this:
#include <iostream>
int main()
{
std::cout << "Hello world!"
return 0;
}Feel free to compile this ill-formed program yourself.
When compiled using Clang, the following error message is generated:
prog.cc:5:31: error: expected ';' after expression
Clang is telling us that on line 5 at the 31st character, the syntax rules require a semicolon, but we did not provide one. Therefore, compilation was halted with an error.
When compiled with Visual Studio instead, the compiler produces this compilation error:
c:\vcprojects\hello.cpp(6): error C2143: syntax error : missing ';' before 'return'
Note that Visual Studio says the error was encountered on line 6 (instead of on line 5). So who is right? Both are, in a way.
Clang knows we conventionally put semicolons at the end of statements, so it is reporting that the error is on line 5 based on the assumption that we will do so. Visual Studio is opting to report the line it was compiling when it determined that the error occurred (on line 6, when it encountered return instead of the expected semicolon).
Key insight
Compilers will sometimes report that an error has occurred on the line after the one where we’d conventionally fix the issue. If you can’t find the error on the line indicated, check the prior line.
To see other different error messages, experiment with deleting characters or even whole lines from the “Hello world” program. Also try restoring the missing semicolon at the end of line 5, and then deleting lines 1, 3, or 4 to see what happens.
Quiz time
The following quiz is meant to reinforce your understanding of the material presented above.
Question #1
What is a statement?
Question #2
What is a function?
Question #3
What is the name of the function that all programs must have?
Question #4
What happens when the program is run?
Question #5
What symbol are statements in C++ often ended with?
Question #6
What is a syntax error?
Question #7
What is the C++ Standard Library?