Why does modifying a `const` value through a non-const pointer lead to undefined behavior but still print different values for the pointer and the original variable?

Modifying a const through a Non-const Pointer

Consider the following code:

<code class="cpp">const int e = 2;

int* w = (int*) &e;          // (1) cast to remove const-ness
*w = 5;                        // (2)

cout << *w << endl;            // (3) outputs 5
cout << e << endl;             // (4) outputs 2

cout << "w = " << w << endl;   // (5) w points to the address of e
cout << "&e = " << &e << endl;</code>

In (1), w points to the address of e. In (2), that value is changed to 5. However, when the values of *w and e are displayed, their values are different. But if you print the pointer w and the address of e, they have the same value.

How come e still contains 2, even if it was changed to 5? Were they stored in a separate location? Or a temporary? But how come the value pointed by w is still the address of e?

Answer

Once you modify a const value, you enter undefined behavior territory. However, to speculate:

• (3) and (4): *w is evaluated at runtime, while e is treated as a compile-time constant. Hence, the different printed values.

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