报错：表达式必须是一个可修改的左值，请问这到底是哪里错掉了？

void* my_memcpy(void* dest, const void* rete, size_t num)
{
assert(dest != NULL);
assert(rete != NULL);
void* ser = dest;
while (num--)
{
(char)dest = (char)rete;
++(char*)dest;
++(char*)rete;
}
return ser;
}

dest+=sizeof(char);
rete+=sizeof(char);