override #define value

Question

4.00/5 (1 vote)

See more:

Hi,

as title said, can we override #define's value

like

C++

#include<stdio.h>
#define LIM 5

// in main function
// scanf x's value and assign it to LIM
// LIM = x;

can we do that ??

Posted 17-Sep-13 8:35am

Muhammad Waqar Siddiqui

Add a Solution

4 solutions

Solution 4

The answers given to your question and this link show why it is better to use a static const than a #define in this case.

http://stackoverflow.com/questions/1637332/static-const-vs-define[^]

However #define can be used to create a macro and this is quite legal though I would not recommend it.

C++

#define LIM (x)

 int x = 5;

 cout << LIM << endl;

 x = 6;

  cout << LIM << endl;

Posted 17-Sep-13 18:14pm

User 59241

Updated 17-Sep-13 18:16pm

v2

Solution 2

No, you cannot override a #define constant with variable at run-time. The reason is simple: #define statements are processed by the compiler -- at compile-time, i.e. long before your program runs. And the compiler inserts those values as constants in your program code.

But you can do the following:

C++

#define LIM 5

void MyFunction ()
{
    int x;

    if (!scanf ("%d", &x) != 1)
        x = LIM;
    ...    
}

Posted 17-Sep-13 8:53am

nv3

Updated 17-Sep-13 8:54am

v2

Comments

ridoy 17-Sep-13 16:39pm

good explanation,5ed!

nv3 17-Sep-13 16:55pm

Thank you!

Solution 1

You cannot do that.

The #define are replaced (textually) in by the pre-compilation phase of the compiler.

Posted 17-Sep-13 8:49am

Maximilien

Comments

ridoy 17-Sep-13 16:39pm

agree,5ed!

Sergey Alexandrovich Kryukov 17-Sep-13 18:33pm

Of course, a 5.
—SA

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

H.Brydon · Accepted Answer · 2013-09-17T17:36:00

As the other solutions have indicated, you can't change a value at runtime, but you can change the compile time definition of a macro during the compile process. For example:

C++

void foo()
{
#define LIM 5
    int zork[LIM]; // defined as size 5

#undef LIM
#define LIM 42
    int blork[LIM];  // defined as size 42

#undef LIM
#define LIM 123

    // At this point, LIM is 123, but zork is still size 5 and blork is still
    // size 42. This does not change during the compile process or during runtime.
#undef LIM
    // At this point, LIM is undefined.
}