Question

I have written 2 programs. Please go through both the programs and help me in understanding why variable ‘i’ and ‘*ptr’ giving different values.

//Program I:
//Assumption: Address of i = 100, address of ptr = 500

int i = 5;
int *ptr = (int *) &i;

*ptr = 99;

cout<<i; // 99
cout<<&i;// 100
cout<<ptr; // 100
cout<<*ptr; // 99
cout<<&ptr; // 500
//END_Program_I===============

//Program II:
//Assumption: Address of i = 100, address of ptr = 500
const int i = 5;
int *ptr = (int *) &i;

*ptr = 99;

cout<<i; // 5
cout<<&i;// 100
cout<<ptr; // 100
cout<<*ptr; // 99
cout<<&ptr; // 500
//END_PROGRAM_II===============

The confusion is: Why variable i still coming as 5, even though *ptr ==99?

Answer 1

In the following three lines, you are modifying a constant:

const int i = 5;
int *ptr = (int *) &i;

*ptr = 99;

This is undefined behavior. Anything can happen. So don’t do it.

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As for what’s happening underneath in this particular case:

Since i is const, the compiler assumes it will not change. Therefore, it simply inlines the 5 to each place where it is used. That’s why printing out i shows the original value of 5.