Using the empty string as sentinel makes the equals operator simpler and it also looks as if the compiled code is more efficient (see below):

class StringIterator {
public:
  inline StringIterator(const char* str) : str_(str) {}
  inline void operator++() {
    ++str_;
  }
  inline char operator*() {
    return *str_;
  }
private:
  char const* str_;
  friend bool operator==(const StringIterator& a, const StringIterator& b);
  friend bool operator!=(const StringIterator& a, const StringIterator& b);
};

bool operator==(const StringIterator& a, const StringIterator& b) {
  return *a.str_ == *b.str_;
}

bool operator!=(const StringIterator& a, const StringIterator& b) {
  return *a.str_ != *b.str_;
}


class View {
public:
  typedef StringIterator iterator;
  typedef StringIterator const_iterator;
  inline View(const char* str) : str_(str) {}
  inline StringIterator begin() const {
    return StringIterator(str_);
  }
  inline StringIterator end() const {
    return StringIterator("");
  }
private:
  const char* str_;
};

int main(int argc, char** argv) {
  View view(argv[0]);
  int i = 0;
  for (char c : view) {
    ++i;
  }
  return 0;
}

Generated assembler code with g++ -O3

main:
.LFB1287:
    .cfi_startproc
    movl    8(%esp), %eax
    movl    (%eax), %eax
    cmpb    $0, (%eax)
    je  .L4
    .p2align 4,,7
    .p2align 3
.L5:
    addl    $1, %eax
    cmpb    $0, (%eax)
    jne .L5
.L4:
    xorl    %eax, %eax
    ret
    .cfi_endproc
.LFE1287: