A function is a reusable sequence of statements designed to do a particular job. Functions you write yourself are called user-defined functions.
A function call is an expression that tells the CPU to execute a function. The function initiating the function call is the caller, and the function being called is the callee or called function. Do not forget to include parenthesis when making a function call.
The curly braces and statements in a function definition are called the function body.
A function that returns a value is called a value-returning function. The return type of a function indicates the type of value that the function will return. The return statement determines the specific return value that is returned to the caller. A return value is copied from the function back to the caller -- this process is called return by value. Failure to return a value from a non-void function will result in undefined behavior.
The return value from function main is called a status code, and it tells the operating system (and any other programs that called yours) whether your program executed successfully or not. By consensus a return value of 0 means success, and a non-zero return value means failure.
Practice DRY programming -- “don’t repeat yourself”. Make use of variables and functions to remove redundant code.
Functions with a return type of void do not return a value to the caller. A function that does not return a value is called a void function or non-value returning function. Void functions can’t be called where a value is required.
A return statement that is not the last statement in a function is called an early return. Such a statement causes the function to return to the caller immediately.
A function parameter is a variable used in a function where the value is provided by the caller of the function. An argument is the specific value passed from the caller to the function. When an argument is copied into the parameter, this is called pass by value.
Function parameters and variables defined inside the function body are called local variables. The time in which a variable exists is called its lifetime. Variables are created and destroyed at runtime, which is when the program is running. A variable’s scope determines where it can be seen an used. When a variable can be seen an used, we say it is in scope. When it can not be seen, it can not be used, and we say it is out of scope. Scope is a compile-time property, meaning it is enforced at compile time.
Whitespace refers to characters used for formatting purposes. In C++, this includes spaces, tabs, and newlines.
A forward declaration allows us to tell the compiler about the existence of an identifier before actually defining the identifier. To write a forward declaration for a function, we use a function prototype, which includes the function’s return type, name, and parameters, but no function body, followed by a semicolon.
A definition actually implements (for functions and types) or instantiates (for variables) an identifier. A declaration is a statement that tells the compiler about the existence of the identifier. In C++, all definitions serve as declarations. Pure declarations are declarations that are not also definitions (such as function prototypes).
Most non-trivial programs contain multiple files.
When two identifiers are introduced into the same program in a way that the compiler or linker can’t tell them apart, the compiler or linker will error due to a naming collision. A namespace guarantees that all identifiers within the namespace are unique. The std namespace is one such namespace.
The preprocessor is a process that runs on the code before it is compiled. Directives are special instructions to the preprocessor. Directives start with a # symbol and end with a newline. A macro is a rule that defines how input text is converted to a replacement output text.
Header files are files designed to propagate declarations to code files. When using the #include directive, the #include directive is replaced by the contents of the included file. When including headers, use angled brackets when including system headers (e.g. those in the C++ standard library), and use double quotes when including user-defined headers (the ones you write). When including system headers, include the versions with no .h extension if they exist.
Header guards prevent the contents of a header from being included more than once into a given code file. They do not prevent the contents of a header from being included into multiple different code files.
Be sure to use your editor’s auto-formatting feature to keep your formatting consistent and make your code easier to read.
Write a single-file program (named main.cpp) that reads two separate integers from the user, adds them together, and then outputs the answer. The program should use three functions:
- A function named “readNumber” should be used to get (and return) a single integer from the user.
- A function named “writeAnswer” should be used to output the answer. This function should take a single parameter and have no return value.
- A main() function should be used to glue the above functions together.
Modify the program you wrote in exercise #1 so that readNumber() and writeAnswer() live in a separate file called “io.cpp”. Use a forward declaration to access them from main().
If you’re having problems, make sure “io.cpp” is properly added to your project so it gets compiled.
Modify the program you wrote in #2 so that it uses a header file (named io.h) to access the functions instead of using forward declarations directly in your code (.cpp) files. Make sure your header file uses header guards.