Convert char array to unsigned char*

Question

Is there a way to convert char[] to unsigned char*?

char buf[50] = "this is a test"; 
unsigned char* conbuf = // what should I add here

Solution 1

Although it may not be technically 100% legal this will work reinterpret_cast<unsigned char*>(buf).

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The reason this is not 100% technically legal is due to section 5.2.10 expr.reinterpret.cast bullet 7.

A pointer to an object can be explicitly converted to a pointer to an object of a different type. original type yields the original pointer value, the result of such a pointer conversion is unspecified.

Which I take to mean that *reinterpret_cast<unsigned char*>(buf) = 'a' is unspecified but *reinterpret_cast<char*>(reinterpret_cast<unsigned char*>(buf)) = 'a' is OK.

Solution 2

Just cast it?

unsigned char *conbuf = (unsigned char *)buf;

Comments

[HolyBlackCat]

If I remember the standard correctly, there is an exception that says that it is legal to use pointer to [[un]signed] char to access memory of an object of any type. I think it makes behavior of your code well-defined.

[Motti]

@HolyBlackCat I don't remember any such wording, if you could supply a reference I'll update the answer.

[HolyBlackCat]

Unforunately I'm unable to find a standard reference. But I found some claims on SO that there is an exception to the strict aliasing rule for char * (which means that "it is legal to use pointer to [[un]signed] char to access memory of an object of any type"). See this: stackoverflow.com/a/99010/2752075 You can use char* for aliasing <...>. The rules allow an exception for char* (including signed char and unsigned char). It's always assumed that char* aliases other types. However this won't work the other way: there's no assumption that your struct aliases a buffer of chars.