Summary  
This chapter explains how to use the C preprocessor’s #define directive to create object macros that perform compile-time text substitution of numeric constants or expressions, highlighting their immutable nature and differences from variables.  

General domain of usage  
Defining symbolic constants and simple inline operations in C code for compile-time substitution.

<!DOCTYPE html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <style>
        .custom-text {
            color: red; 
            font-size: 50px;
        }
    </style>
</head>
<body>
    <p class="custom-text">picture_1</p>
</body>
</html>


Each **#define** line consists of three parts:

<!DOCTYPE html>

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <style>
        .custom-text {
            color: red; 
            font-size: 50px;
        }
    </style>
</head>
<body>
    <p class="custom-text">picture_2</p>
</body>
</html>

The body of a macro can be not only a single numeric value, but also other macros or entire expressions:

I2luY2x1ZGUgPHN0ZGlvLmg+CiNkZWZpbmUgVFdPIDIgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgIC8vIG1hY3JvcyB3aXRoIHZhbHVlLgojZGVmaW5lIFJFU1VMVCBUV08qVFdPICAgICAgICAgICAgICAgICAgICAgICAvLyBUV08gaXMgcmVwbGFjZWQgYnkgdGhlIG51bWJlciAyLgojZGVmaW5lIFBSSU5UIHByaW50ZigicmVzdWx0ID0gJWQiLCBSRVNVTFQpOwoKaW50IG1haW4oKSAKewogICBQUklOVCAKICAgcmV0dXJuIDA7Cn0=

<!DOCTYPE html>

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <style>
        .custom-text {
            color: red; 
            font-size: 50px;
        }
    </style>
</head>
<body>
    <p class="custom-text">picture_3</p>
</body>
</html>

Every time the compiler encounters “TWO”, it will simply substitute its value `2`.

**The macro is immutable**. Once defined, it is a permanent textual substitution within the file or scope where it is defined.

**Why do we need this? Why not just use variables?**

What is better to use?:
- If the value never changes, **const is better**, because it is safer;
- If you need to substitute a simple number or text, you can use a macro.


1. Create an object macro `add`, the body of the macro is an addition operator `+`. 
2. Use a macro to add any two numbers.


You will discover the secret of compilation using the console and learn how to compile programs manually. Get to know the preprocessor better and learn how to create your own macros of varying complexity. Take your programming skills to the next level with a set of preprocessor directives that will make your programs more understandable, convenient, and make compilation flexible!

Dive into the compilation process, see how to do compilation yourself using terminal commands.

Get acquainted with macros and their varieties. You will learn how to create/delete object and functional macros, and also master new tricks for working with strings .

Create conditions for different compilation scenarios using conditional preprocessor directives.

Dive into advanced preprocessor capabilities that allow you to precisely control compilation, interact with the compiler, and create flexible, manageable code using directives and built-in macros.

Symbolic Constant

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