C++ How to invoke an object's method in a different file

Bob Walance · Nov 17, 2023

I have created an object for the class Timers in timer_functions.cpp, and I need to run a method associated with that object ( test_method() ) from interrupts.cpp.

I've tried many things but have been unsuccessful. Any ideas would be appreciated. Here is some sample code:

classes.hpp

C++:

   #pragma once
   class Timers {
   public:
     void test_method(void);
   private:
     int testVariable;
   };

classes.cpp

C++:

#include classes.hpp
   void Timers::test_method(void) {
     testVariable++;
   }

timer_functions.cpp

C++:

#include classes.hpp
   Timers myTimerObject; // create the object

interrupts.cpp

C++:

#include classes.hpp
   extern Timers myTimerObject; // this doesn't eliminate the build error
   void timer_interrupt(void) {
     myTimerObject.test_method(); // trying to run the objects method, but BUILD ERROR
   }

Vanadium 50 · Nov 17, 2023

I'm not clear on what you are trying to do, and how things are split up. Normally, if you have a class foo, there is a header foo.hh and the actual code in foo.cc. If I have code bar.cc, it includes foo.hh, and the linker links foo.o and bar.o.

I don't see how your snippets match to this, but you seem to be creating an instance of an object in one file and using it in another. That shouldn't work - it shouldn't even compile, because the object isn't even declared in the second file. (And if it is declared, how does the compiler know it is supposed to be the same as in the first?)

Normally code would create the object and pass it to other code through a pointer to that object. The same code that created it should destroy it when it is no longer needed,

Bob Walance · Nov 17, 2023

Vanadium 50 said:

I'm not clear on what you are trying to do, and how things are split up. Normally, if you have a class foo, there is a header foo.hh and the actual code in foo.cc. If I have code bar.cc, it includes foo.hh, and the linker links foo.o and bar.o.

I don't see how your snippets match to this, but you seem to be creating an instance of an object in one file and using it in another. That shouldn't work - it shouldn't even compile, because the object isn't even declared in the second file. (And if it is declared, how does the compiler know it is supposed to be the same as in the first?)

Normally code would create the object and pass it to other code through a pointer to that object. The same code that created it should destroy it when it is no longer needed,

I don't want to create and destroy the instance for each periodic timer interrupt. While this would probably work, it seems messy.

A pointer to the instance could be created in timer_functions.cpp, but how would it get passed to the timer interrupt?

strangerep · Nov 17, 2023

Bob Walance said:

I don't want to create and destroy the instance for each periodic timer interrupt. While this would probably work, it seems messy.

A pointer to the instance could be created in timer_functions.cpp, but how would it get passed to the timer interrupt?

If you intend to have only a single (global) instance of Timers, then you can use a "static" instance. In that case, read up on static class variables and static member fns.

Of course, if you do indeed intend that, then why call it "Timers" (plural) instead of "Timer" singular?

Bob Walance · Nov 17, 2023

strangerep said:

If you intend to have only a single (global) instance of Timers, then you can use a "static" instance. In that case, read up on static class variables and static member fns.

Wow. That works!

Yes, a single instance of the class (really it's no instance) is just fine for my application.

Thank you for the simple solution.

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