lvalue required on incrementing a void pointer even after proper casting

Question

I am implementing memset() method. Below is the code snippet:

void my_memset(void* ptr, int n, size_t size)
{
    unsigned int i;

    for( i = 0; i < size; ++i, ++(char*)ptr )
            *(char*)ptr = n;
}

I am getting the error as:

prog.cpp: In function ‘void my_memset(void*, int, size_t)’:
prog.cpp:8: error: lvalue required as increment operand

When i change the type of ptr from void* to int*, it compiles successfully.

Why it is asking for lvalue?

Answer 1

void my_memset(void* ptr, int n, size_t size)
{
    char *cptr = (char *)ptr;
    char *limit = cptr + size;

    for(; cptr < limit; cptr++) {
        *cptr = n;
    }
}

you can't increment through a cast like that, and you shouldn't want to.

Answer 2

The result of a type-casting is not an lvalue. Say,

float f = 3.14;
++((int)f);

here we're applying ++ operator to an int value, but we don't have an int variable to increment. So in general case, type-casting cannot produce lvalues.

Answer 3

You can't for the same reason you can't do this:

++someFunction();

or

 ++(a + b);

The cast is an expression and the results of expressions don't have writable storage.