#define and just declaration

[Eric Hong]

What's the difference between

#define BLAH_BLAH_BLAH 123

and

int BLAH_BLAH_BLAH = 123;
?

Thanks.

[ModShop]

A define directive is used during compile time, almost like a variable, and is treated as a constant at run time. A variable is around during run time, in memory, rather than being a constant value.

#define NUM 5

cout << NUM;

will turn into:

cout << 5;

Whereas:

const int NUM = 5;

cout << NUM;

Will store the value of NUM in RAM at run time.

[PanGalactic]

With the second version I can do:

BLAH_BLAH_BLAH = 124;

I can't do that with the first.

Now, the real question is: What's the difference between "#define BLAH 123" and "const int BLAH = 123;"? ;-)

[DSLq5Di1]

http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.7

[Eric Hong]

Thanks everyone!

[shacktar]

@PanGalactic If the compiler optimizes const int BLAH by placing the value directly into the machine instructions, then not much.

[PanGalactic]

@shacktar:

namespace foo
{
#define BLAH_1 123
const int BLAH_2 = 123;
}

Notice a difference here?

[shacktar]

I was assuming something like this.

#define BLAH_1 123
const int BLAH_2 = 123;

#include <iostream>
using namespace std;

int main()
{
    cout<<BLAH_1<<endl;
    cout<<BLAH_2<<endl;

    return 0;
}

In the above case, the compiler can optimize for memory and never put BLAH_2 into RAM. I didn't stop to think about scoping, which of course is the major difference :)