Function declaration inside or outside the class

C++ is object oriented, in the sense that it supports the object oriented paradigm for software development.

However, differently from Java, C++ doesn't force you to group function definitions in classes: the standard C++ way for declaring a function is to just declare a function, without any class.

If instead you are talking about method declaration/definition then the standard way is to put just the declaration in an include file (normally named .h or .hpp) and the definition in a separate implementation file (normally named .cpp or .cxx). I agree this is indeed somewhat annoying and requires some duplication but it's how the language was designed (the main concept is that C++ compilation is done one unit at a time: you need the .cpp of the unit being compiled and just the .h of all the units being used by the compiled code; in other words the include file for a class must contain all the information needed to be able to generate code that uses the class). There are a LOT of details about this, with different implications about compile speed, execution speed, binary size and binary compatibility.

For quick experiments anything works... but for bigger projects the separation is something that is practically required (even if it may make sense to keep some implementation details in the public .h).

Note: Even if you know Java, C++ is a completely different language... and it's a language that cannot be learned by experimenting. The reason is that it's a rather complex language with a lot of asymmetries and apparently illogical choices, and most importantly, when you make a mistake there are no "runtime error angels" to save you like in Java... but there are instead "undefined behavior daemons".

The only reasonable way to learn C++ is by reading... no matter how smart you are there is no way you can guess what the committee decided (actually being smart is sometimes even a problem because the correct answer is illogical and a consequence of historical heritage.)

Just pick a good book or two and read them cover to cover.

The first defines your member function as an inline function, while the second doesn't. The definition of the function in this case resides in the header itself.

The second implementation would place the definition of the function in the cpp file.

Both are semantically different and it is not just a matter of style.

Function definition is better outside the class. That way your code can remain safe if required. The header file should just give declarations.

Suppose someone wants to use your code, you can just give him the .h file and the .obj file (obtained after compilation) of your class. He does not need the .cpp file to use your code.

That way your implementation is not visible to anyone else.

The "Inside the class" (I) method does the same as the "outside the class" (O) method.

However, (I) can be used when a class is only used in one file (inside a .cpp file). (O) is used when it is in a header file. cpp files are always compiled. Header files are compiled when you use #include "header.h".

If you use (I) in a header file, the function (Fun1) will be declared every time you include #include "header.h". This can lead to declaring the same function multiple times. This is harder to compile, and can even lead to errors.

Example for correct usage:

File1: "Clazz.h"

//This file sets up the class with a prototype body. 

class Clazz
{
public:
    void Fun1();//This is a Fun1 Prototype. 
};

File2: "Clazz.cpp"

#include "Clazz.h" 
//this file gives Fun1() (prototyped in the header) a body once.

void Clazz::Fun1()
{
    //Do stuff...
}

File3: "UseClazz.cpp"

#include "Clazz.h" 
//This file uses Fun1() but does not care where Fun1 was given a body. 

class MyClazz;
MyClazz.Fun1();//This does Fun1, as prototyped in the header.

File4: "AlsoUseClazz.cpp"

#include "Clazz.h" 
//This file uses Fun1() but does not care where Fun1 was given a body. 

class MyClazz2;
MyClazz2.Fun1();//This does Fun1, as prototyped in the header. 

File5: "DoNotUseClazzHeader.cpp"

//here we do not include Clazz.h. So this is another scope. 
class Clazz
{
public:
    void Fun1()
    {
         //Do something else...
    }
};

class MyClazz; //this is a totally different thing. 
MyClazz.Fun1(); //this does something else.