sstring: add sstring::front()

as required by the C++20 standard, std::string should provide

constexpr const charT& front() const;
constexpr charT& front();

so let's add them to sstring. so that sstring can be used in lieu of
std::string, when appropriate.

see also https://eel.is/c++draft/basic.string#string.access

Signed-off-by: Kefu Chai <[email protected]>

include/seastar/core/sstring.hh

@@ -461,6 +461,29 @@ public:
         replace(p, p, beg, end);
     }
 
+
+    /**
+     *  Returns a read/write reference to the data at the first
+     *  element of the string.
+     *  This function shall not be called on empty strings.
+     */
+    reference
+    front() noexcept {
+        assert(!empty());
+        return *str();
+    }
+
+    /**
+     *  Returns a  read-only (constant) reference to the data at the first
+     *  element of the string.
+     *  This function shall not be called on empty strings.
+     */
+    const_reference
+    front() const noexcept {
+        assert(!empty());
+        return *str();
+    }
+
     /**
      *  Returns a read/write reference to the data at the last
      *  element of the string.

tests/unit/sstring_test.cc

@@ -51,6 +51,12 @@ BOOST_AUTO_TEST_CASE(test_add_literal_to_sstring) {
     BOOST_REQUIRE_EQUAL("x" + sstring("y"), sstring("xy"));
 }
 
+BOOST_AUTO_TEST_CASE(test_front) {
+    sstring s("abcde");
+    BOOST_CHECK_EQUAL(s.front(), 'a');
+    BOOST_CHECK_EQUAL(std::as_const(s).front(), 'a');
+}
+
 BOOST_AUTO_TEST_CASE(test_find_sstring) {
     BOOST_REQUIRE_EQUAL(sstring("abcde").find('b'), 1u);
     BOOST_REQUIRE_EQUAL(sstring("babcde").find('b',1), 2u);