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Join Date: Mar 2008

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theausum

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Help me.

Apr 6th, 2008

What is the difference between #define n 10 and const int n=10

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Join Date: Feb 2008

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BlackJavaBean BlackJavaBean is offline

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Re: Help me.

Apr 6th, 2008

#define is preprocessor directive - it literally tells the preprocessor to replace n with 10 . const is actual C++ - it says that n is a constant of type int that is assigned the value 10. They'll do about the same thing, but are used for different reasons. In this case, n is a horrible choice for the variable name if #define is used because it could allow the preprocessor to insert 10's where you didn't mean for them to be. Another strike against the preprocessor is that it will define the constant globally - where as const follows scope restrictions native to C++.

Last edited by BlackJavaBean; Apr 6th, 2008 at 5:21 am. Reason: Added tags and update explanation

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Laiq Ahmed Laiq Ahmed is offline

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Re: Help me.

Apr 6th, 2008

difference b/w const and #define (macro).

1. Type Checking:- macros are not type safe where as const are type safe. since the macros are replaced by definition at the time of preprocessing (which is done before compilation). the storage area of macros is not defined where as const has static storage area.

for example

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#define n  10
int main ()
{
       // what would be the behavior of the below code.
       cout<< &n<<endl; 
       return 0;
}
#define n  10
int main ()
{
       // what would be the behavior of the below code.
       cout<< &n<<endl; 
       return 0;
}

2. macros are error prune
for example the below code

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
 
#define max(a,b)       ((a>b)?a:b)
 
int main ()
{
     int a = 2;
     int b = 3;
     cout<<max(a++, b)<<endl;
     return 0;
}
#define max(a,b)       ((a>b)?a:b)

int main ()
{
     int a = 2;
     int b = 3;
     cout<<max(a++, b)<<endl;
     return 0;
}

check the above code. the proper solution to above is to use templates rather than macros for specifying generic operatoins.
[see. Effective C++ for more details.].

so the conclusion is that use consts & templates where necessary don't use macros.