prepend operator

Makefile

 HDRS = src/BBBParser.h src/SourceFile.h
 BUILD = build
 OBJS = $(patsubst %,$(BUILD)/%.o,$(SRCS))
-CFLAGS = -g
+CFLAGS = -g -Wall -Wextra -Wno-unused-parameter
 
 $(BUILD)/%.cc.o: %.cc $(HDRS)
 	@mkdir -p $(@D)

sample.bbb

 	$other_files
 
 cflags :=-g -O3 -DTITLE="Hello World"
-cflags += HELLO nope
+cflags += these values will be appended
+cflags =+ these values will be prepended

src/BBBParser.cc

 		skip(); // '+'
 		skip(); // '='
 		return Operator::PLUS_EQUALS;
+	} else if (peek() == '=' && ch2 == '+') {
+		skip(); // '='
+		skip(); // '+'
+		return Operator::EQUALS_PLUS;
 	}
 
 	return Operator::NONE;
 	}
 }
 
-void BBBParser::error(std::string msg) {
+[[noreturn]] void BBBParser::error(std::string msg) {
 	throw BBBParseError(std::to_string(line_) + ":" + std::to_string(ch_) + ": " + msg);
 }
 
 			vars[key] = std::move(values);
 			values.clear();
 			break;
+
 		case Operator::PLUS_EQUALS:
 			{
 				auto &vec = vars[key];
 			}
 			values.clear();
 			break;
+
+		case Operator::EQUALS_PLUS:
+			{
+				auto &vec = vars[key];
+				vec.reserve(vec.size() + values.size());
+				for (size_t i = 0; i < vec.size(); ++i) {
+					values.push_back(std::move(vec[i]));
+				}
+				vec = std::move(values);
+			}
+			values.clear();
+			break;
+
 		case Operator::NONE:
 			break;
 		}

src/BBBParser.h

 	enum class Operator {
 		COLON_EQUALS,
 		PLUS_EQUALS,
+		EQUALS_PLUS,
 		NONE,
 	};
 
 	Operator readOperator();
 	void skip(char);
 	void skip() { get(); }
-	void error(std::string);
+	[[noreturn]] void error(std::string);
 
 	void skipWhitespaceLine();
 	void skipWhitespace();