What is the difference between a definition and a declaration?

Question

The meaning of both eludes me.

Answer 1

You can declare a variable without defining it in the same line. For instance:

int x; //this is a "declaration" of a new integer "x".

x = 9; //defines this variable x to have a value of 9.

You can also declare a function without defining it. You can do this by declaring functions at the top of a single-file program so that the compiler does not give compilation errors and then you define the function later. Also, for programs with multiple files you normally have a header file where your function declarations are and then you define these functions in a different file.

Here is an example of the first scenario:

int foo(int a, int b); // this is the function declaration for "foo'

int main() {

//Some code calling foo(...);

}

int foo(int a, int b) { //this begins the function definition for the function "foo"

a++;

b++;

return a + b;

}

I hope this helps

Answer 2

definition pertains mostly to types. You can DEFINE a TYPE of data structure which consists of primitive types: integer, float, character, string, boolean; No memory is reserved, it's just a definition.

declaring a variable of a primitive or data structure type actually reserves memory for the data,

EX.

// DEFINES a data structure type and a pointer to it's type

typedef struct _TDataRec

{

int iNum;

double flAmt;

char zStr[25];

} * TDataRec;

#define DATA_REC_SIZE ((sizeof(struct _TDataRec)) //size of the data structure

int x; //reserves memory of 1 integer with a name of x

float y; // reserve memory of 1 double precision floating point with a number of y

char zBuff[25]; // declares an array of 25 characters, a.k.a string, with the name zBuff;

struct _TDataRec myDataRec; //reserves memory for one DataRec structure;

TDataRec dataRecPtr = &myDataRec; // reserves memory for the address of a DataRec and

// stores the address of myDataRec in it;