difference between definition, declaration & prototype

Searching for content on this forum before creating a thread I stumbled on const function which post #2-3 caught my eye.

Originally Posted by grumpy

A "function definition" has a specific meaning in C++, which I suspect you have misunderstood.

Originally Posted by neutron_star

sorry its function prototype.

I then made a few searches on the separate terms but for most interesting sites, the definitions seemed to contradict one another. I then finally tried my luck searching what I really what to know: c+++function+definition+declaration+prototype+diff erence OR compare OR comparison OR versus OR VS ; that is the difference between definition, declaration & prototype. I have my own assumptions but I won't share it because they'd very likely hamper the issue instead of helping it.

prototype applies specifically to functions, and in that context, a prototype is the same as a declaration:

Code:

int getSomeNumber();

a class can have a (forward) declaration:

Code:

class someClass;

but the class definition would be like so:

Code:

class someClass
{
public:
  someClass();
};

now a function definition is the part that actually contains the code, like from our above example:

Code:

int getSomeNumber()
{
    return 3; // 3 is the most random number available
}

a function definition is also sometimes referred to as its implementation, and defining/implementing a standalone function - that is, not a class member - effectively also declares it.

note that in my class example that the constructor is declared, but there is no code, so it is not defined, although the class can be considered defined at this point. the definition/implementation of the constructor might look like this:

Code:

someClass::someClass()
{
    // do some initialization
}

A function declaration is a prototype.

A function definition is a function with a heading and a body.

That's all.

Sometimes, a function declaration can be a definition, depending on whether you wrote a prototype, and if you wrote a function in the right place. One of the first things you learn about functions is that they need to be declared before they are called. Well, if you define a function before it is called, that works too.

In common conversation it is better to ask for clarification if you are confused. Sometimes the distinction between the terms is ignored because it isn't exactly ambiguous what somebody means.

[edit]More generally, a definition is considered to be the line of code that allocates resources for the thing. A declaration will just reserve a name.[/edit]

Rather than try and describe the grammar to you formally, here's an example of a function definition:

Code:

void foo() {}

A function definition is also a declaration of the function, but while there can only be one definition of a given function, there can be many declarations of the same function.

Here's an example of a function declaration that is not a function definition:

Code:

void foo();

A function declaration that is not a function definition is also known as a function prototype when it is used as a forward declaration (i.e., it is declared before some code that calls the function or takes a pointer to the function).