Is const_cast safe?

Question

I can't find much information on const_cast. The only info I could find (on Stack Overflow) is:

The const_cast<>() is used to add/remove const(ness) (or volatile-ness) of a variable.

This makes me nervous. Could using a const_cast cause unexpected behavior? If so, what?

Alternatively, when is it okay to use const_cast?

Answer 1

const_cast is safe only if you're casting a variable that was originally non-const. For example, if you have a function that takes a parameter of a const char *, and you pass in a modifiable char *, it's safe to const_cast that parameter back to a char * and modify it. However, if the original variable was in fact const, then using const_cast will result in undefined behavior.

void func(const char *param, size_t sz, bool modify)
{
    if(modify)
        strncpy(const_cast<char *>(param), sz, "new string");
    printf("param: %s\n", param);
}

...

char buffer[16];
const char *unmodifiable = "string constant";
func(buffer, sizeof(buffer), true);  // OK
func(unmodifiable, strlen(unmodifiable), false); // OK
func(unmodifiable, strlen(unmodifiable), true);  // UNDEFINED BEHAVIOR