What is the difference between declaration and definition?

The MSDN article can be very confusing. Ignore the part that says there can be more than one declaration; it is there to be technically accurate but it is misleading. It is just saying that you can put the same declaration in more than one place, but note that it would (must) be the same everywhere.

A definition is memory allocated and initialized based on a declaration.

Note that it is also very confusing to attempt to differentiate the two for fundamental types. In other words, plain types such as "int" have implicit declarations.

Also note that the PHP language is very confused. The implementers of that language misuse the terms horribly. The C++ language standard is consistent with the explanation above.

Declaration:  Declaration means we tell compiler that what will be the type and nature of my function or method .That means we are just introducing it or we are just introducing the signature or prototype of our function.

Ex-

public int sum(int,int);

Here i have declared one method namely sum whose nature is that it will take two arguments of integer type an it will return one integer type value.


Definition: Definition means specifying the functionality of our function. Defining means to provide a body to a function that what our function is going to do.

Ex-
public int sum(int a,int b)
{
return(a+b); //it will find the sum of two numbers
} 

hi,

Declaration: it means that something is declaring such as a class, function or variable. You don't say anything about what that class or function looks like, you just say that you will say what it looks like.we can say This function or this variable exists somewhere, and here is what it should look like.

Definition: You define how something is implemented, such as a class, function or variable, i.e. you say what it actually is.we can say says: "Make this variable here" or "Make this function here."

example Delegate is Declaration where as event is Defination.

hope this will help you.

Check out this MSDN page which explains the difference quite well:

http://msdn.microsoft.com/en-us/library/0e5kx78b.aspx