Use end() in constructor

to_string.hpp

@@ -32,9 +32,9 @@ class to_string_t {
      * Constructs the object, filling `buf` with the string representation of N.
      */
     constexpr to_string_t() noexcept {
-        if (N != 0) {
-            auto ptr = buf + sizeof(buf) / sizeof(buf[0]);
+        auto ptr = end();
         *--ptr = '\0';
+        if (N != 0) {
             for (auto n = N < 0 ? -N : N; n; n /= base)
                 *--ptr = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"[n % base];
             if (N < 0)