#define MYCLASS_H ?

[bnilsson]

In many of the Qt example headers it starts with something like this:

#ifndef MYCLASS_H
#define MYCLASS_H
 
#include ....
 
class myClass : public QSomeClass 
{
...
};

#ifndef MYCLASS_H
#define MYCLASS_H

#include ....

class myClass : public QSomeClass 
{
...
};

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What is the meaning and purpouse of the MYCLASS_H definitions?
I have noticed that it does not compile if they don't have the same name as the class in the file.