Teach Yourself Borland C++ in 14 Days - portal

Wading In Deeper
So how do the source files all get tied together? First, the compiler compiles each source file
(.cpp) into an object file (.obj). After each module has been compiled, the linker links all the
object files together to make a single executable file (the .exe). The linker also may link in
other needed files such as resource files (.res) and library files (.lib).
The declarations for classes and structures are often kept in a separate file called a
header file. Headers have a filename extension of .h or .hpp. (I touched on headers
briefly when I discussed the iostream class in Day 1, “Getting Your Feet Wet.”) A header file
should contain only class, structure, and function declarations. You should never put any
code statements in a header.
NEW TERM
NOTE
There is an exception to the rule that no code should be placed in
headers. You may put inline functions in headers. An inline function is a
special function in terms of the way the compiler generates code for the
function. You’ll learn more about inline functions on Day 4, “Totally
Immersed: C++ Classes and Object-Oriented Programming,” when I
discuss classes.
Once you have created a header file for a class or structure, you can include that header in
any source code module that needs to see the class or structure declaration. To do that you
use the #include directive:
#include “structur.h”
When you use the #include directive, it is as if the contents of the file being included
were pasted into the source file at that point. Listing 2.4, in the next section, contains a
program that uses the #include directive. The header file used in Listing 2.4 is contained in
Listing 2.5.
TIP
Header files typically implement a sentry to ensure that the header is
included only once for a program. A sentry essentially tells the compiler,
“I’ve already been included once, so don’t include me again.” A
sentry looks like this:
#ifndef _MYCLASS_H
#define _MYCLASS_H
class MyClass {
// class declared here
};
#endif
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