The difference between #include .h and just stating class A; in C++ -

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this question has answer here:

i'm analying operating systems project school , came across header file:

//kernelev.h

#ifndef _kernelev_h  #define _event_h_  typedef unsigned char ivtno;  class thread; class pcb; class kernelsem;  class kernelev { public:      kernelev (ivtno ivtno);      ~kernelev();      int wait(int maxtimetowait);      void signal();

[...]

now, when writing complete definitions of these methods (kernelev, ~kernelev, wait , signal), used attributes of classes thread, pcb , kernelsem. difference between introducing instance #include thread.h; #include kernelsem.h; , declaring classes this: class thread; there differences in data access rights? or it's somehow different?

thanks help, hope question clear enough.

first, note if introduce classes, won't able use methods;

class thread;  thread x; // compile error: size of x unknown thread* x; // ok // set x valid thread, maybe parameter x->signal(); // compile error

but makes no difference whether declarations in header or included in file. is, replace include line copy of header , work fine (every line in above example valid). there many reasons not to, however. ease of maintenance top issue, along readability , modularity. less applicable compiler caching (so take longer compile)


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