Deep understanding of C language Define

In the previous article, there is a pre-compiled directive that says # include, and a common precompiled directive: #define. Its surface meaning is defined, which is usually said to "define constants", but its real function is to replace;

Such as:

#define SUCCESS 1

The whole sentence is to define a macro substitution where success is the macro name and the following is the value;

How to use:

Using success directly in the next program will represent 1. But success is not a variable, and although it looks like a constant, its value does not change. But the real constant is to follow the compiler, that is, in the later debugging you can print a constant, such as: const a=3; You can print out the value of a. But the # define thing is replaced when it is precompiled, that is, you can not see the success at compile time, it is replaced directly with 1, this is one of the differences with the const ... This is the use of macros, which is also used for macro substitution.

What is the role of # define in the following context?

Role 1: Convenient for programmers to detect.

Because in many cases, for example you may use 1, if you write directly 1, then many places may appear 1, you follow-up, see 1, it is difficult to immediately know what the meaning of the 1, in the context of the situation, so let oneself become troublesome. In this case you can use these macros to express, this can be "at a glance", as you write success, you can suddenly know that the operation is the meaning of success.

Role 2: Easy maintenance.

Such as: can be used to define the array capacity

#define MAX_SIZE 100

This can be used for subsequent use:

int a[max_size];

Later, if the capacity is small, directly modify the macro, then the use of the place is followed by the latest, because you will recompile ...

Role 3: Simplifying the wording

Such as: it can be defined as:

#define UNSIGNED short UST

Then you can write it in the following way:

UST A;

Role 4: It can actually do other languages.

Such as: Chinese programming language, easy language

such as: Here in Chinese programming:

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Execution effect:

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The reason for this is written in the header file:

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Because, according to the previous section, you know. h is freely definable ...

This is one of the reasons why the C language can do other programming languages, such as: C language to write PHP interpreter, you can create a PHP programming language ...

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Deep understanding of C language Define