definition/declaration/...

I'm unsure when should I use words 'declaration', 'definition', 'implementation'..

Could someone explain what is the proper way of using words above (what exactly each of them means)?

Thanks a lot

A lot of these words are used interchangably, so it's kind of hard to pin down what people mean when they use those words. Here's generally what I think they are supposed to mean:

Declaration:

Code:

---

int x; // Variable
int f(int a); // Function

---

Implementation:

Code:

---

int f(int a) // Function
{
    fire_death_ray();
}

---

Definition:

Uh....

Code:

---

typedef SOMEINT int // :D

---

lol

I suppose if you declare a variable as extern in a header file and then redeclare it normally in a cpp file, then that would count as a declaration vs an implementation.

Edit: I'm going to get eaten alive in this topic for this post. I can see it coming. :D But I'm pretty tired at the moment (24+ hours ftw!), so that's my excuse. :D

Code:

---

void f( ); // declaration

void f( ) // definition
{
    return;
}

---

Implementation basically is only code that represents a concept. You can implement a Matrix class, a Graphics engine, a compression algorithm..

http://c-faq.com/sx1/index.html#declarations
http://c-faq.com/decl/decldef.html

And maybe some differences between C and C++ here:
http://david.tribble.com/text/cdiffs.htm

Code:

---

int x; // Variable

---

That is both a declaration and a definition (unless contained inside a class or struct or union).

A declaration declares something by saying "this thing exists". A declaration might also be a definition, but it doesn't have to be. (Some people might say that technically if the declaration is a definition, it isn't a declaration.) Here are some declarations that are not also definitions:

Code:

---

class Foo; // forward declaration of a class

extern int x; // extern variable declaration
extern int y; // extern variable declaration

void bar(); // function declaration

---

A definition defines the thing. For a variable, an instance of that variable is created. For a function, the implementation of that function is given. For a class, the members of the class are defined.

Code:

---

class Foo // class definition
{
public:
  Foo(); // class member function declaration
private:
  static int data; // class static member variable declaration
};

Foo::Foo() { } // class member function definition
int Foo::data = 0; // class static member variable definition

int x = 0; // variable definition
int y; // variable definition

void bar() // function definition
{
  cout << x << ',' << y << '\n';
}

---