Difference between using define and variable

Question

If I have the code below:

#define POUND_PER_DOLLAR  73
int nPound = 4 * POUND_PER_DOLLAR;

AND

int POUND_PER_DOLLAR = 122;
int nPound = 4 * POUND_PER_DOLLAR;

Are there instances where usage of one is more suited than the other?

A good compiler should evaluate the second and avoid useless stack usage, anyway the macro is **not** typesafe — Marco A., Aug 01 '14 at 13:08
This might be of interest: [static const vs #define](http://stackoverflow.com/questions/1637332/static-const-vs-define) — t.niese, Aug 01 '14 at 13:09
What happens if the exchange rate changes for your customer? — Neil Kirk, Aug 01 '14 at 13:17
Why is you code shouting so much? It's unseemly. [Stop the constant SHOUTING](http://accu.org/index.php/journals/1923) — Jonathan Wakely, Aug 01 '14 at 13:57

score 3 · Accepted Answer · answered Aug 01 '14 at 13:07

If you need the address, you need a variable:

void foo(int *);

foo(&POUND_PER_DOLLAR);         // must be an lvalue

If you need a constant expression, a macro (or at least a constant) will work:

char array[POUND_PER_DOLLAR];   // must be a constant expression

However, the most appropriate construction is probably a constant:

const int kPoundPerDollar = 73;
int nPound = 4 * kPoundPerDollar;

void bar(const int *);
bar(&kPoundPerDollar);                 // works
char c[kPoundPerDollar];               // also works

template <const int * P> struct X {};
X<&kPoundPerDollar> x;                 // also works

I'd also add something about type safety – Marco A. Aug 01 '14 at 13:09 — Marco A., Aug 01 '14 at 13:09

score 3 · Answer 2 · answered Aug 01 '14 at 13:08

3

Neither. The #define is not type-safe, the int is non-const. This is the constant you're looking for :

int const POUND_PER_DOLLAR = 122;

answered Aug 01 '14 at 13:08

Quentin

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except it looks fugly in ALL_CAPS and there's no good reason for that. [Stop the constant SHOUTING](http://accu.org/index.php/journals/1923). – Jonathan Wakely Aug 01 '14 at 13:58

Samer · Answer 3 · 2014-08-01T14:13:43.460

#define identifier replacement

When the preprocessor encounters this directive, it replaces any occurrence of identifier in the rest of the code by replacement. This replacement can be an expression, a statement, a block or simply anything. The preprocessor does not understand C++ proper, it simply replaces any occurrence of identifier by replacement.

Disadvantages of using #define: method,

the preprocessor does not understand any c++, it only replaces it, so you are taking your own risk
It makes it harder for debugging, if you are using a debugger and you want to check the values of your variables
You Have to be careful of redefinition of your macro

Difference between using define and variable

3 Answers3