What is definition? What is a statement? What is the difference between them?

What is the definition ？

What is the definition ： The so-called definition is ( compiler ) Create an object , Allocate a piece of memory for this object and give it a name , This name is what we often call variable name or object name . But pay attention to , Once the name matches this memory , They live and die together , Never give up . And the location of this memory cannot be changed . A variable or object is in a certain area （ For example, in a function , Global, etc ） Can only be defined once , If defined more than once , The compiler will prompt you to define the same variable or object repeatedly .

int i;

What is a statement ？

What is a statement ： It has two meanings , as follows ：
First meaning ： Tell compiler , The name has been matched to a piece of memory , The following code uses variables or objects that are defined elsewhere . Statements can appear more than once .

extern int	i;

Second meaning ： Tell compiler , I made a reservation for my name , It can no longer be used as a variable name or object name anywhere else . For example, you put a book on a seat in the self-study room of the Library , It indicates that this seat has been reserved , No one is allowed to use this seat anymore . In fact, you are not sitting in this seat at this time . The most typical example of this kind of declaration is the declaration of function parameters , for example ：

void fun(int i, char c);

What's the difference ？

The definition is to allocate memory space .
Declare that there is no need to allocate memory space .