Why Does Modifying a `const` Variable Through a Non-Const Pointer Seem to Work, but Doesn\'t Actually Change Its Value?

Modifying a const Through a Non-Const Pointer

In C , a const variable cannot be modified once initialized. However, in certain scenarios, it may appear that a const variable has been altered. Consider the following code:

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

int* w = (int*)&e;        // (1)
*w = 5;                   // (2)

cout << *w << endl;          // (3)
cout << e << endl;             // (4)</code>

If you run this code, you'll notice an unexpected behavior:

5
2

Even though *w was changed to 5 in (2), e still holds its original value of 2. This seemingly paradoxical behavior stems from the following factors:

• (1) Dereferencing a const pointer (w) allows for modification.
• (2) The modified value is stored in the memory location pointed to by w, which in this case is the memory location where e is stored.
• However, the compiler optimizes the code, treating e as a compile-time constant and not evaluating it at runtime.

As a result, when *w is evaluated at runtime, it returns the modified value (5). However, when e is evaluated at compile time, its original value (2) is used.

This behavior is known as undefined behavior in C . Modifying a const variable directly or indirectly leads to unpredictable consequences, and caution should be exercised in such situations.

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