Preprocessor directive: Why Identifier for header file with #define is not same as header file name [duplicate]

Question

Why inside myheader.h, we write:

#ifndef MYHEADER_H
#define MYHEADER_H
...
#endif

Since, Identifiers are case-sensitive (lowercase and uppercase letters are distinct), and every character is significant. How the above is valid? It should be

#ifndef myheader.h
#define myheader.h
...
#endif

What is the purpose of the underscores? What if I have 2 separate header files with names: MyHeader.h and myheader.h, How does the preprocessor distinguish between these?

Read: What are the rules about using an underscore in a C++ identifier? — WhozCraig
– WhozCraig, Commented Feb 25, 2017 at 10:53

Cheers and hth. - Alf · Accepted Answer · 2017-02-25 10:50:02Z

1

The leading underscore followed by uppercase indicates that it's either a name defined by the C++ implementation, because this form is reserved for the implementation, or it's a name defined by someone who has seen this in system headers and just copy-cat adopted it without understanding it.

answered Feb 25, 2017 at 10:50

Cheers and hth. - Alf

146k15 gold badges218 silver badges342 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Porcupine Over a year ago

So for user defined header files, we should not include the leading underscores?

Cheers and hth. - Alf Over a year ago

@Nikhil: Right. Also avoid other reserved forms, which are (1) two successive underscores anywhere, or, (2) leading underscore in a name in the global namespace.

Collectives™ on Stack Overflow

Preprocessor directive: Why Identifier for header file with #define is not same as header file name [duplicate]

1 Answer 1

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Linked

Related