"UnConsting" const value via pointer

First, sorry for possible question redundancy.

Doing some little experiments with C/C++ pointers in GCC I encountered this somewhat weird behaviour when bypassing constantness of value at the pointer address.

#include <iostream>
int main()
{
    using namespace std;

    const double number = 100;
//bypassing constantess of pointed-to value
    d开发者_运维知识库ouble * pointer_to_value = (double *) &number;
    *pointer_to_value += 200;

    cout << "Adress of number: " << &number << "\tValue of number: " << number << endl <<
    "   Pointer value: " << pointer_to_value << "\tDereferencing pointer: " << *pointer_to_value;

    return 0;
}

I would expect both form of checking the value yielding same results. Location of value is same in both cases. Program generates following output, however:

Adress of number: 0x22ff30 Value of number: 100 Pointer value: 0x22ff30 Dereferencing pointer: 300

Anyone capable of explaining? Thanks in advance.

It's undefined behaivor.

It's irrelevant why exactly it happens (actually because the compiler inlines the value).

"UnConsting” const value via pointer is a Undefined Behavior.
So it is not posible to define a behavior not defined by the Standard.

Compiler optimization. Compiler doesn't expect you to try and trick it like that, it knows that the value is const, so it just cached it. Try to compile it without any optimization, and see if it makes any difference.

Generally the meaning of const is:

constant - the object shall not be modified. Attempt to do so results in undefined behavior. On most of the compilers it is compile-time error.

Compiler optimization. You can overcome that by adding volatile keyword to the variable.

#include <iostream>
int main()
{
    using namespace std;

    volatile const double number = 100;
    //bypassing constantess of pointed-to value
    double * pointer_to_value = (double *) &number;
    *pointer_to_value += 200;

    cout << "Adress of number: " << &number << "\tValue of number: " << number << endl <<
    "   Pointer value: " << pointer_to_value << "\tDereferencing pointer: " << *pointer_to_value;

    return 0;
}

My guess is that gcc has done some optimizations on your behalf, replacing the reference to << number with << 100. Should be possible to verify by looking at the generated asm code.