Chapter 19.4

clox/src/object.h

@@ -25,7 +25,9 @@ struct ObjString {
   char *chars;
 };
 
+ObjString *takeString(char *chars, int length);
 ObjString *copyString(const char *chars, int length);
+void printObject(Value value);
 
 static inline bool isObjType(Value value, ObjType type) {
   return IS_OBJ(value) && AS_OBJ(value)->type == type;

clox/src/value.c

@@ -1,6 +1,8 @@
 #include <stdio.h>
+#include <string.h>
 
 #include "memory.h"
+#include "object.h"
 #include "value.h"
 
 void initValueArray(ValueArray *array) {
@@ -37,6 +39,9 @@ void printValue(Value value) {
   case VAL_NUMBER:
     printf("%g", AS_NUMBER(value));
     break;
+  case VAL_OBJ:
+    printObject(value);
+    break;
   }
 }
 
@@ -50,6 +55,12 @@ bool valuesEqual(Value a, Value b) {
     return true;
   case VAL_NUMBER:
     return AS_NUMBER(a) == AS_NUMBER(b);
+  case VAL_OBJ: {
+    ObjString *aString = AS_STRING(a);
+    ObjString *bString = AS_STRING(b);
+    return aString->length == bString->length &&
+           memcmp(aString->chars, bString->chars, aString->length) == 0;
+  }
   default:
     return false; /* Unreachable */
   }

clox/src/vm.c

@@ -1,9 +1,12 @@
 #include <stdarg.h>
 #include <stdio.h>
+#include <string.h>
 
 #include "common.h"
 #include "compiler.h"
 #include "debug.h"
+#include "memory.h"
+#include "object.h"
 #include "vm.h"
 
 VM vm;
@@ -43,6 +46,20 @@ static bool isFalsey(Value value) {
   return IS_NIL(value) || (IS_BOOL(value) && !AS_BOOL(value));
 }
 
+static void concatenate() {
+  ObjString *b = AS_STRING(pop());
+  ObjString *a = AS_STRING(pop());
+
+  int length = a->length + b->length;
+  char *chars = ALLOCATE(char, length + 1);
+  memcpy(chars, a->chars, a->length);
+  memcpy(chars + a->length, b->chars, b->length);
+  chars[length] = '\0';
+
+  ObjString *result = takeString(chars, length);
+  push(OBJ_VAL(result));
+}
+
 static InterpretResult run() {
 #define READ_BYTE() (*vm.ip++)
 #define READ_CONSTANT() (vm.chunk->constants.values[READ_BYTE()])
@@ -96,9 +113,19 @@ static InterpretResult run() {
     case OP_LESS:
       BINARY_OP(BOOL_VAL, <);
       break;
-    case OP_ADD:
-      BINARY_OP(NUMBER_VAL, +);
+    case OP_ADD: {
+      if (IS_STRING(peek(0)) && IS_STRING(peek(1))) {
+        concatenate();
+      } else if (IS_NUMBER(peek(0)) && IS_NUMBER(peek(1))) {
+        double b = AS_NUMBER(pop());
+        double a = AS_NUMBER(pop());
+        push(NUMBER_VAL(a + b));
+      } else {
+        runtimeError("Operands must be two numbers or two strings.");
+        return INTERPRET_RUNTIME_ERROR;
+      }
       break;
+    }
     case OP_SUBTRACT:
       BINARY_OP(NUMBER_VAL, -);
       break;