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Sep 23rd, 2009

Urgent :decrement of read-only location

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Hi everyone,

Can anyone tells me what this error means:

error: decrement of read-only location.

thanks

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aomran

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Sep 23rd, 2009

Re: Urgent :decrement of read-only location

My guess is that it means just what it says.

Do you suppose you could post relevant code or some sort of context?

Dave Sinkula

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long time no c

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Sep 23rd, 2009

Re: Urgent :decrement of read-only location

I will post the relevant code shortly, but I need more explanation of what decrement of read only location. location of what and how it has been decremented.

aomran

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Sep 23rd, 2009

Re: Urgent :decrement of read-only location

well your compile should give a line number.....
What you are not saying is if the error is at run-time or at compile time.

Compile time:

you may have written something like this

c++ Syntax (Toggle Plain Text)
const int X(40);       // set X to 40
X--;                          // set X to 39 not allowed.
const int X(40);       // set X to 40
X--;                          // set X to 39 not allowed.

Run time, you may have written:

c++ Syntax (Toggle Plain Text)
int *Ptr(10);   // intending *Ptr to point to a location 10  but 
                      // actually pointing it to location 
// error here:
(*Ptr)--;         // Maybe you intended to write *Ptr--
int *Ptr(10);   // intending *Ptr to point to a location 10  but 
                      // actually pointing it to location 
// error here:
(*Ptr)--;         // Maybe you intended to write *Ptr--

Actually the last error is a little difficult to find, without using the debugger or a lot of print statements.

StuXYZ

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Sep 23rd, 2009

Re: Urgent :decrement of read-only location

Like the others already stated this error comes from trying to manipulate a constant. This should make you wonder a few things including:

Why am I trying to manipulate a constant? It totally beats the point of making something constant if you're going to decrement it later.

If you have to decrement the constant then why not just remove the const and make it a normal variable?

However, if you really need to do this and there's some logical reason for it then there's always const_cast<>. Take a look at my example:

C++ Syntax (Toggle Plain Text)
int main()
{
   const int myVar = 5;
   int* j = const_cast<int*>(&myVar);
   *j = *j - 1;
   cout << *j << ": as you can see it's been decremented to 4.\n";
 
   return 0;
}
int main()
{
   const int myVar = 5;
   int* j = const_cast<int*>(&myVar);
   *j = *j - 1;
   cout << *j << ": as you can see it's been decremented to 4.\n";

   return 0;
}

necrolin

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Sep 23rd, 2009

Re: Urgent :decrement of read-only location

Or maybe you have a const method and you try to decrement a member variable in this method.

Last edited by jencas; Sep 23rd, 2009 at 5:13 am.

jencas

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Sep 23rd, 2009

Re: Urgent :decrement of read-only location

I actually got these two messages

dheap.h: In member function ‘void Dheap<Process>::deleteMin() const’:
dheap.h:191: error: decrement of read-only location

aomran

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Sep 23rd, 2009

Re: Urgent :decrement of read-only location

The "const" at the end of your first error line is your problem. The member function is declared constant so it won't allow you to make any changes to variables in the function.

necrolin

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Sep 24th, 2009

Re: Urgent :decrement of read-only location

thanks guys it works

aomran

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