I've seen similar questions asked yet they still do not make sense to my ape brain.
Here is an example. If I declared a function in a header file named Bob.h: void PrintSomething(); and in the .cpp file I say: void MyClass::PrintSomething(){std::cout << "Hello";} . I've seen people in another .cpp file for example Frank.cpp, only include the Bob.h header which just has the declaration (No code inside it) and not the .cpp with the code but then what blows my mind is when they call the PrintSomething() function in Frank.cpp it uses the code from Bob.cpp and prints "Hello". How? How does it print "Hello" which was added in the .cpp file when I've only included the .h file which doesn't say anything about "Hello", its just a declaration? I've looked through the compile process and linking process too but it just doesn't stick.
On top of which if I were to now say in my Frank.cpp file: void MyClass::PrintSomething(){std::cout << "Bye";} and included the Bob.h file in my main.cpp and called the PrintSomething() function would it print "Hello" or "Bye"? Is the computer psychic or something? This concept is the one thing I am not grasping in my C++ learning journey.
Thanks in advance.
The moment you include Bob.h the compiler has everything it needs to know about PrintSomething(), it only need a declaration of the function. Frank.cpp does not need to know about Bob.cpp which defines PrintSomething().
All of your individual cpp files output object files generated by the compiler. These in themselves don't do much until they're all glued together, this is the linker's responsibility.
The linker takes all your object files and fills in the missing parts:
Linker talk:
Hey, I see that
Frank.objusesPrintSomething()and I can't see its definition in that object file.Let's check the other object files..
Upon inspecting
Bob.objI can see that this contains a usable definition forPrintSomething(), let's use that.
This is of course simplified but that's what a linker does in short.
After this is done you get your usable executable.
on top of which if I were to now say in my Frank.cpp file:
void MyClass::PrintSomething(){std::cout << "Bye";}and included theBob.hfile in mymain.cppand called thePrintSomething()function would it print "Hello" or "Bye"? Is the computer psychic or something?
The linker would find 2 definitions of PrintSomething() and would emit an error, it has no way to know what definition is the right one to pick.
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