Why Does Modifying a Constant through a Non-const Pointer Produce Undefined Behavior?

Modifying a Constant through a Non-const Pointer: Delving into Undefined Behavior

In the provided code snippet, you express confusion over the behavior of modifying a constant integer (e) through a non-const pointer (w):

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

int* w = (int*) &e;  // Casting to remove const-ness
*w = 5;            // Modifying the value pointed to by w</code>

After making this modification, you observe that:

• Displaying the value pointed to by w (cout << w << endl;) outputs 5, as expected.
• However, displaying the value of e (*cout << e << endl;) surprisingly outputs 2, its original value.

You further notice that the address pointed to by w is the same as the address of e (cout << "w = " << w << endl;cout << "&e = " << &e << endl;). This leads you to question how e remains unchanged despite the modification made through w.

This behavior stems from the undefined behavior that arises when modifying a constant through a non-const pointer. Once you make such a modification, the code enters undefined behavior territory, where it becomes unpredictable and depends on specific implementation details.

In this case, it appears that the modification through w affects a temporary copy of e at runtime, while the original e remains unchanged. The reason for this is that e is treated as a compile-time constant, and its value is hardcoded into the binary code. Therefore, any runtime modifications to w will not affect the original e.

This behavior is specific to the implementation used and should not be relied upon. Modifying constant data through non-const pointers is considered a bad practice and should be avoided. The correct approach is to use a non-const reference or create a non-const copy of the data you intend to modify.

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