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2.10 — Introduction to the preprocessor

Translation and the preprocessor

When you compile your code, you might expect that the compiler compiles the code exactly as you’ve written it. This actually isn’t the case.

Prior to compilation, the code file goes through a phase known as translation. Many things happen in the translation phase to get your code ready to be compiled (if you’re curious, you can find a list of translation phases here [1]).

The most noteworthy of the translation phases involves the preprocessor. The preprocessor is best thought of as a separate program that manipulates the text in each code file.

When the preprocessor runs, it scans through the code file (from top to bottom), looking for preprocessor directives. Preprocessor directives (often just called directives) are instructions that start with a # symbol and end with a newline (NOT a semicolon). These directives tell the preprocessor to perform specific particular text manipulation tasks. Note that the preprocessor does not understand C++ syntax -- instead, the directives have their own syntax (which in some cases resembles C++ syntax, and in other cases, not so much).

The output of the preprocessor goes through several more translation phases, and then is compiled. Note that the preprocessor does not modify the original code files in any way -- rather, all text changes made by the preprocessor happen temporarily in-memory each time the code file is compiled.

In this lesson, we’ll discuss what some of the most common preprocessor directives do.

Includes

You’ve already seen the #include directive in action (generally to #include <iostream>). When you #include a file, the preprocessor replaces the #include directive with the preprocessed contents of the included file, which is then compiled.

Consider the following program:

When the preprocessor runs on this program, the preprocessor will replace #include <iostream> with the preprocessed contents of the file named “iostream”.

Since #include is almost exclusively used to include header files, we’ll discuss #include in more detail in the next lesson (when we discuss header files in more detail).

Macro defines

The #define directive can be used to create a macro. In C++, a macro is a rule that defines how input text is converted into replacement output text.

There are two basic types of macros: object-like macros, and function-like macros.

Function-like macros act like functions, and serve a similar purpose. We will not discuss them here, because their use is generally considered dangerous, and almost anything they can do can be done by a normal function.

Object-like macros can be defined in one of two ways:

#define identifier
#define identifier substitution_text

The top definition has no substitution text, whereas the bottom one does. Because these are preprocessor directives (not statements), note that neither form ends with a semicolon.

Object-like macros with substitution text

When the preprocessor encounters this directive, any further occurrence of the identifier is replaced by substitution_text. The identifier is traditionally typed in all capital letters, using underscores to represent spaces.

Consider the following program:

The preprocessor converts the above into the following:

Which, when run, prints the output My name is: Alex.

We recommend avoiding these kinds of macros altogether, as there are better ways to do this kind of thing. We discuss this more in lesson 4.13 -- Const, constexpr, and symbolic constants [2].

Object-like macros without substitution text

Object-like macros can also be defined without substitution text.

For example:

Macros of this form work like you might expect: any further occurrence of the identifier is removed and replaced by nothing!

This might seem pretty useless, and it is useless for doing text substitution. However, that’s not what this form of the directive is generally used for. We’ll discuss the uses of this form in just a moment.

Unlike object-like macros with substitution text, macros of this form are generally considered acceptable to use.

Conditional compilation

The conditional compilation preprocessor directives allow you to specify under what conditions something will or won’t compile. There are quite a few different conditional compilation directives, but we’ll only cover the three that are used by far the most here: #ifdef, #ifndef, and #endif.

The #ifdef preprocessor directive allow the preprocessor to check whether an identifier has been previously #defined. If so, the code between the #ifdef and matching #endif is compiled. If not, the code is ignored.

Consider the following program:

Because PRINT_JOE has been #defined, the line cout << "Joe" << endl; will be compiled. Because PRINT_BOB has not been #defined, the line cout << "Bob" << endl; will be ignored.

#ifndef is the opposite of #ifdef, in that it allows you to check whether an identifier has NOT been #defined yet.

This program prints “Bob”, because PRINT_BOB was never #defined.

#if 0

One more common use of conditional compilation involves using #if 0 to exclude a block of code from being compiled (as if it were inside a comment block):

The above code only prints “Joe”, because “Bob” and “Steve” were inside an #if 0 block that the preprocessor will exclude from compilation.

This provides a convenient way to “comment out” code that contains multi-line comments.

Object-like macros don’t affect other preprocessor directives

Now you might be wondering:

Since we defined PRINT_JOE to be nothing, how come the preprocessor didn’t replace PRINT_JOE in #ifdef PRINT_JOE with nothing?

Macros only cause text substitution for normal code. Other preprocessor commands are ignored. Consequently, the PRINT_JOE in #ifdef PRINT_JOE is left alone.

For example:

In actuality, the output of the preprocessor contains no directives at all -- they are all resolved/stripped out before compilation, because the compiler wouldn’t know what to do with them.

The scope of defines

Directives are resolved before compilation, from top to bottom on a file-by-file basis.

Consider the following program:

Even though it looks like #define MY_NAME “Alex” is defined inside function foo, the preprocessor won’t notice, as it doesn’t understand C++ concepts like functions. Therefore, this program behaves identically to one where #define MY_NAME “Alex” was defined either before or immediately after function foo. For general readability, you’ll generally want to #define identifiers outside of functions.

Once the preprocessor has finished, all defined identifiers from that file are discarded. This means that directives are only valid from the point of definition to the end of the file in which they are defined. Directives defined in one code file do not have impact on other code files in the same project.

Consider the following example:

function.cpp:

main.cpp:

The above program will print:

Not printing!

Even though PRINT was defined in main.cpp, that doesn’t have any impact on any of the code in function.cpp (PRINT is only #defined from the point of definition to the end of main.cpp). This will be of consequence when we discuss header guards in a future lesson.


2.11 -- Header files [3]
Index [4]
2.9 -- Naming conflicts and the std namespace [5]