Conversion from const int to int giving strange results.Can anyone explain the reason for the strange results

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When I tried below code I got strange results.I am trying to change value of constant by using the pointers.But when I output the results pointer value and the original variable variable value its giving two different values.Can anyone explain what exactly happens when explicit conversion take place?

C++

int main()
{
    int *p ;
    const int a = 20;
    p=(int *)&a;
    *p = *p +10;
    cout<<"p is"<<*p<<"\na is"<<a;
}

output:

p is 30 
a is 20

Posted 6-Jun-15 4:53am

krish0969

Updated 6-Jun-15 5:19am

Frankie-C

v2

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Ashish Tyagi 40 6-Jun-15 12:26pm

Because you ignored a warning :-)

2 solutions

Solution 2

You are taking the address of a const int and then cast it to a non-const pointer. And you are wondering why the program is behaving strangely? ...

Posted 6-Jun-15 12:21pm

nv3

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Frankie-C · Accepted Answer · 2015-06-06T05:18:00

The C++ compiler have very strong optimization when dealing with const's. To spare time and space the optimizer never allocate any instance of a const int, but use to supply directly the value where it is requested in the more efficient way for that specific condition.
So when you ask for a pointer to a constant the compiler creates an instance of the constant value in memory (having a life scope compliant with the scope of the request) and pass the pointer of this constant to the piece of code that requested it.
In the successive part of your code where you ask for the constant value the compiler knows that is much more efficient to pass the value instead of to retrieve the value previously allocated.
Practically even if you modify the value accessed by pointer you cannot modify another instance of the value.
The C++ compiler treats the const values almost as a #define, but with more flexibility.
BTW your code is absolutely incorrect because you cannot modify a constant. The incorrect usage that you made exposed this side effect (it should be a bug, but C++ purists could shoot us if we make this affirmation :D).