Where is the undefined behavior when using const_cast<>?

Question

If I do:

const char* const_str = \"Some string\";

char* str = const_cast(const_str); // (1)

str[0] = \"P\"; // (2)

Where (wh

Answer 1

You are attempting to modify a constant string which the compiler may have put into a read-only section of the process. This is better:

char str[32];
strcpy(str, "Some string");
str[0] = "P";

Answer 2

Line (2) has undefined behaviour. The compiler is at liberty to place constants in read-only memory (once upon a time in Windows this would have been a "data segment") so writing to it might cause your program to terminate. Or it might not.

Having to cast const-ness away when calling a poorly-defined library function (non-const parameter which should be const) is, alas, not unusual. Do it, but hold your nose.