#define constant declaration

In C or C++ languages, pre-processing statements that begin with # are called preprocessing statements.

#define has several uses. As I asked, #defineAB finds all the same spelling as A in the code before compiling and replaces it with B.

After the replacement is finished, proceed with the compilation.

For example,

#define TAX 0.5f
float t=TAX;

The above code will be compiled after being replaced as shown below.

float t=0.5f;

When defined as #defineAB, A is usually referred to as a constant (or macro constant, also known as a symbol constant).

As I asked, the text A is replaced with B itself.