feat: use character literal in character_either

using `std::vector` for this scanner requires many times of memory
allocation and free.

include/toml11/fwd/scanner_fwd.hpp

@@ -98,22 +98,10 @@ class character_either final : public scanner_base
 
   public:
 
-    explicit character_either(std::initializer_list<char_type> cs) noexcept
-        : chars_(std::move(cs))
-    {
-        assert(! this->chars_.empty());
-    }
-
     template<std::size_t N>
     explicit character_either(const char (&cs)[N]) noexcept
-        : chars_(N-1, '\0')
+        : value_(cs), size_(N-1) // remove null character at the end
-    {
+    {}
-        static_assert(N >= 1, "");
-        for(std::size_t i=0; i+1<N; ++i)
-        {
-            chars_.at(i) = char_type(cs[i]);
-        }
-    }
     ~character_either() override = default;
 
     region scan(location& loc) const override;
@@ -122,12 +110,11 @@ class character_either final : public scanner_base
 
     scanner_base* clone() const override;
 
-    void push_back(const char_type c);
-
     std::string name() const override;
 
   private:
-    std::vector<char_type> chars_;
+    const char* value_;
+    std::size_t size_;
 };
 
 // ----------------------------------------------------------------------------

include/toml11/impl/scanner_impl.hpp

@@ -87,8 +87,9 @@ TOML11_INLINE region character_either::scan(location& loc) const
 {
     if(loc.eof()) {return region{};}
 
-    for(const auto c : this->chars_)
+    for(std::size_t i=0; i<this->size_; ++i)
     {
+        const auto c = char_type(this->value_[i]);
         if(loc.current() == c)
         {
             const auto first = loc;
@@ -101,30 +102,32 @@ TOML11_INLINE region character_either::scan(location& loc) const
 
 TOML11_INLINE std::string character_either::expected_chars(location&) const
 {
-    assert( ! chars_.empty());
+    assert( this->value_ );
+    assert( this->size_ != 0 );
 
     std::string expected;
-    if(chars_.size() == 1)
+    if(this->size_ == 1)
     {
-        expected += show_char(chars_.at(0));
+        expected += show_char(char_type(value_[0]));
     }
-    else if(chars_.size() == 2)
+    else if(this->size_ == 2)
     {
-        expected += show_char(chars_.at(0)) + " or " + show_char(chars_.at(1));
+        expected += show_char(char_type(value_[0])) + " or " +
+                    show_char(char_type(value_[1]));
     }
     else
     {
-        for(std::size_t i=0; i<chars_.size(); ++i)
+        for(std::size_t i=0; i<this->size_; ++i)
         {
             if(i != 0)
             {
                 expected += ", ";
             }
-            if(i + 1 == chars_.size())
+            if(i + 1 == this->size_)
             {
                 expected += "or ";
             }
-            expected += show_char(chars_.at(i));
+            expected += show_char(char_type(value_[i]));
         }
     }
     return expected;
@@ -135,20 +138,16 @@ TOML11_INLINE scanner_base* character_either::clone() const
     return new character_either(*this);
 }
 
-TOML11_INLINE void character_either::push_back(const char_type c)
-{
-    chars_.push_back(c);
-}
-
 TOML11_INLINE std::string character_either::name() const
 {
     std::string n("character_either{");
-    for(const auto c : this->chars_)
+    for(std::size_t i=0; i<this->size_; ++i)
     {
+        const auto c = char_type(this->value_[i]);
         n += show_char(c);
         n += ", ";
     }
-    if( ! this->chars_.empty())
+    if(this->size_ != 0)
     {
         n.pop_back();
         n.pop_back();

include/toml11/impl/syntax_impl.hpp

@@ -59,7 +59,7 @@ TOML11_INLINE non_ascii::non_ascii(const spec& s) noexcept
 
 TOML11_INLINE character_either wschar(const spec&)
 {
-    return character_either{char_type(' '), char_type('\t')};
+    return character_either(" \t");
 }
 
 TOML11_INLINE repeat_at_least ws(const spec& s)
@@ -172,7 +172,7 @@ TOML11_INLINE sequence dec_int(const spec& s)
         return character_in_range(char_type('1'), char_type('9'));
     };
     return sequence(
-            maybe(character_either{char_type('-'), char_type('+')}),
+            maybe(character_either("+-")),
             either(
                 sequence(
                     digit19(),
@@ -222,7 +222,7 @@ TOML11_INLINE sequence oct_int(const spec&)
 TOML11_INLINE sequence bin_int(const spec&)
 {
     const auto digit01 = []() {
-        return character_either{char_type('0'), char_type('1')};
+        return character_either("01");
     };
     return sequence(
             literal("0b"),
@@ -274,8 +274,8 @@ TOML11_INLINE sequence fractional_part(const spec& s)
 TOML11_INLINE sequence exponent_part(const spec& s)
 {
     return sequence(
-            character_either{char_type('e'), char_type('E')},
+            character_either("eE"),
-            maybe(character_either{char_type('+'), char_type('-')}),
+            maybe(character_either("+-")),
             zero_prefixable_int(s)
         );
 }
@@ -291,9 +291,9 @@ TOML11_INLINE sequence hex_floating(const spec& s)
     // - 0x(int)p[+-](int)
 
     return sequence(
-            maybe(character_either{char_type('+'), char_type('-')}),
+            maybe(character_either("+-")),
             character('0'),
-            character_either{char_type('x'), char_type('X')},
+            character_either("xX"),
             either(
                 sequence(
                     repeat_at_least(0, hexdig(s)),
@@ -305,8 +305,8 @@ TOML11_INLINE sequence hex_floating(const spec& s)
                     maybe(character('.'))
                 )
             ),
-            character_either{char_type('p'), char_type('P')},
+            character_either("pP"),
-            maybe(character_either{char_type('+'), char_type('-')}),
+            maybe(character_either("+-")),
             repeat_at_least(1, character_in_range('0', '9'))
         );
 }
@@ -322,7 +322,7 @@ TOML11_INLINE either floating(const spec& s)
                 )
             ),
             sequence(
-                maybe(character_either{char_type('-'), char_type('+')}),
+                maybe(character_either("+-")),
                 either(literal("inf"), literal("nan"))
             )
         );
@@ -371,8 +371,8 @@ TOML11_INLINE sequence local_time(const spec& s)
 TOML11_INLINE either time_offset(const spec& s)
 {
     return either(
-            character_either{'Z', 'z'},
+            character_either("zZ"),
-            sequence(character_either{'+', '-'},
+            sequence(character_either("+-"),
                      repeat_exact(2, digit(s)),
                      character(':'),
                      repeat_exact(2, digit(s))
@@ -385,7 +385,7 @@ TOML11_INLINE sequence full_time(const spec& s)
 }
 TOML11_INLINE character_either time_delim(const spec&)
 {
-    return character_either{'T', 't', ' '};
+    return character_either("Tt ");
 }
 TOML11_INLINE sequence local_datetime(const spec& s)
 {
@@ -401,16 +401,19 @@ TOML11_INLINE sequence offset_datetime(const spec& s)
 
 TOML11_INLINE sequence escaped(const spec& s)
 {
-    character_either escape_char{
+    const auto escape_char = [&s] {
-        '\"','\\', 'b', 'f', 'n', 'r', 't'
-    };
         if(s.v1_1_0_add_escape_sequence_e)
         {
-        escape_char.push_back(char_type('e'));
+            return character_either("\"\'bfnrte");
         }
+        else
+        {
+            return character_either("\"\'bfnrt");
+        }
+    };
 
     either escape_seq(
-            std::move(escape_char),
+            escape_char(),
             sequence(character('u'), repeat_exact(4, hexdig(s))),
             sequence(character('U'), repeat_exact(8, hexdig(s)))
         );

include/toml11/parser.hpp

@@ -2679,7 +2679,7 @@ guess_number_type(const location& first, const context<TC>& ctx)
             if('0' <= c && c <= '9')
             {
                 return err(make_syntax_error("bad datetime: missing T or space",
-                    character_either{'T', 't', ' '}, loc, std::string(
+                    character_either("Tt "), loc, std::string(
                     "Hint: valid  : 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999\n"
                     "Hint: invalid: 1979-05-27T7:32:00, 1979-05-27 17:32\n")));
             }