diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8fa0a9a9..7eeb0059 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -135,6 +135,10 @@ jobs:
         run: |
           make stats
 
+      - name: cxxtest
+        run: |
+          make cxxtest
+
       - name: test
         if: ${{ matrix.config.configType != 'examples' && matrix.config.configType != 'tcc' }}
         run: |
@@ -211,6 +215,14 @@ jobs:
           build\${{matrix.buildType}}\qjs.exe examples\test_point.js
           build\${{matrix.buildType}}\run-test262.exe -c tests.conf
           build\${{matrix.buildType}}\function_source.exe
+      - name: Set up Visual Studio shell
+        uses: egor-tensin/vs-shell@v2
+        with:
+          arch: ${{ matrix.arch == 'x64' && 'x64' || 'x86' }}
+      - name: cxxtest
+        run: |
+          cl.exe /DJS_NAN_BOXING=0 /Zs cxxtest.cc
+          cl.exe /DJS_NAN_BOXING=1 /Zs cxxtest.cc
 
   windows-msvc-vs2019:
     runs-on: windows-2019
@@ -228,6 +240,14 @@ jobs:
       - name: stats
         run: |
           build\${{matrix.buildType}}\qjs.exe -qd
+      - name: Set up Visual Studio shell
+        uses: egor-tensin/vs-shell@v2
+        with:
+          arch: ${{ matrix.arch == 'x64' && 'x64' || 'x86' }}
+      - name: cxxtest
+        run: |
+          cl.exe /DJS_NAN_BOXING=0 /Zs cxxtest.cc
+          cl.exe /DJS_NAN_BOXING=1 /Zs cxxtest.cc
 
   windows-clang:
     runs-on: windows-latest
@@ -244,6 +264,10 @@ jobs:
       - name: stats
         run: |
           build\${{matrix.buildType}}\qjs.exe -qd
+      - name: cxxtest
+        run: |
+          clang-cl.exe /DJS_NAN_BOXING=0 /Zs cxxtest.cc
+          clang-cl.exe /DJS_NAN_BOXING=1 /Zs cxxtest.cc
       - name: test
         run: |
           cp build\${{matrix.buildType}}\fib.dll examples\
@@ -315,6 +339,14 @@ jobs:
       run: |
         make stats
         ldd build/qjs
+    - name: cxxtest
+      if: ${{ matrix.sys != 'clang64' }}
+      run: |
+        make cxxtest
+    - name: cxxtest
+      if: ${{ matrix.sys == 'clang64' }}
+      run: |
+        make cxxtest CXX=clang++
     - name: test
       run: |
         make test
@@ -395,6 +427,10 @@ jobs:
       - name: stats
         run: make stats
 
+      - name: cxxtest
+        run: |
+          make cxxtest
+
       - name: test
         run: make test
 
@@ -467,6 +503,9 @@ jobs:
       - name: build
         run: |
           make
+      - name: cxxtest
+        run: |
+          make cxxtest
       - name: test
         run: |
           make test
@@ -484,6 +523,9 @@ jobs:
       - name: build
         run: |
           make
+      - name: cxxtest
+        run: |
+          make cxxtest
       - name: test
         run: |
           make test
diff --git a/Makefile b/Makefile
index dda3af07..7b3ec75b 100644
--- a/Makefile
+++ b/Makefile
@@ -83,6 +83,12 @@ distclean:
 stats: $(QJS)
 	$(QJS) -qd
 
+# implicitly .PHONY because it doesn't generate output
+cxxtest: CXXFLAGS+=-std=c++11 -fsyntax-only -Wall -Wextra -Werror -Wno-unused-parameter
+cxxtest: cxxtest.cc quickjs.h
+	$(CXX) $(CXXFLAGS) -DJS_NAN_BOXING=0 $<
+	$(CXX) $(CXXFLAGS) -DJS_NAN_BOXING=1 $<
+
 test: $(QJS)
 	$(RUN262) -c tests.conf
 
@@ -110,4 +116,4 @@ unicode_gen: $(BUILD_DIR)
 libunicode-table.h: unicode_gen
 	$(BUILD_DIR)/unicode_gen unicode $@
 
-.PHONY: all debug fuzz install clean codegen distclean stats test test262 test262-update test262-check microbench unicode_gen $(QJS) $(QJSC)
+.PHONY: all cxxtest debug fuzz install clean codegen distclean stats test test262 test262-update test262-check microbench unicode_gen $(QJS) $(QJSC)
diff --git a/cxxtest.cc b/cxxtest.cc
new file mode 100644
index 00000000..6488533d
--- /dev/null
+++ b/cxxtest.cc
@@ -0,0 +1,17 @@
+// note: file is not actually compiled, only checked for C++ syntax errors
+#include "quickjs.h"
+
+int main(void)
+{
+    JSRuntime *rt = JS_NewRuntime();
+    JSContext *ctx = JS_NewContext(rt);
+    JS_FreeValue(ctx, JS_NAN);
+    JS_FreeValue(ctx, JS_UNDEFINED);
+    JS_FreeValue(ctx, JS_NewFloat64(ctx, 42));
+    // not a legal way of using JS_MKPTR but this is here
+    // to have the compiler syntax-check its definition
+    JS_FreeValue(ctx, JS_MKPTR(JS_TAG_UNINITIALIZED, 0));
+    JS_FreeContext(ctx);
+    JS_FreeRuntime(rt);
+    return 0;
+}
diff --git a/quickjs.h b/quickjs.h
index 1e331504..12111f80 100644
--- a/quickjs.h
+++ b/quickjs.h
@@ -61,9 +61,11 @@ typedef uint32_t JSAtom;
    - string contents is either pure ASCII or is UTF-8 encoded.
  */
 
+/* Overridable purely for testing purposes; don't touch. */
+#ifndef JS_NAN_BOXING
 #if INTPTR_MAX < INT64_MAX
-/* Use NAN boxing for 32bit builds. */
-#define JS_NAN_BOXING
+#define JS_NAN_BOXING 1 /* Use NAN boxing for 32bit builds. */
+#endif
 #endif
 
 enum {
@@ -90,7 +92,7 @@ enum {
 #define JS_FLOAT64_NAN NAN
 #define JSValueConst JSValue /* For backwards compatibility. */
 
-#if defined(JS_NAN_BOXING)
+#if defined(JS_NAN_BOXING) && JS_NAN_BOXING
 
 typedef uint64_t JSValue;
 
@@ -174,13 +176,41 @@ typedef struct JSValue {
 #define JS_VALUE_GET_FLOAT64(v) ((v).u.float64)
 #define JS_VALUE_GET_PTR(v) ((v).u.ptr)
 
+/* msvc doesn't understand designated initializers without /std:c++20 */
+#ifdef __cplusplus
+static inline JSValue JS_MKPTR(int64_t tag, void *ptr)
+{
+    JSValue v;
+    v.u.ptr = ptr;
+    v.tag = tag;
+    return v;
+}
+static inline JSValue JS_MKVAL(int64_t tag, int32_t int32)
+{
+    JSValue v;
+    v.u.int32 = int32;
+    v.tag = tag;
+    return v;
+}
+static inline JSValue JS_MKNAN(void)
+{
+    JSValue v;
+    v.u.float64 = JS_FLOAT64_NAN;
+    v.tag = JS_TAG_FLOAT64;
+    return v;
+}
+/* provide as macros for consistency and backward compat reasons */
+#define JS_MKPTR(tag, ptr) JS_MKPTR(tag, ptr)
+#define JS_MKVAL(tag, val) JS_MKVAL(tag, val)
+#define JS_NAN             JS_MKNAN() /* alas, not a constant expression */
+#else
+#define JS_MKPTR(tag, p)   (JSValue){ (JSValueUnion){ .ptr = p }, tag }
 #define JS_MKVAL(tag, val) (JSValue){ (JSValueUnion){ .int32 = val }, tag }
-#define JS_MKPTR(tag, p) (JSValue){ (JSValueUnion){ .ptr = p }, tag }
+#define JS_NAN             (JSValue){ (JSValueUnion){ .float64 = JS_FLOAT64_NAN }, JS_TAG_FLOAT64 }
+#endif
 
 #define JS_TAG_IS_FLOAT64(tag) ((unsigned)(tag) == JS_TAG_FLOAT64)
 
-#define JS_NAN (JSValue){ (JSValueUnion){ .float64 = JS_FLOAT64_NAN }, JS_TAG_FLOAT64 }
-
 static inline JSValue __JS_NewFloat64(double d)
 {
     JSValue v;
@@ -900,7 +930,8 @@ static inline JSValue JS_NewCFunctionMagic(JSContext *ctx, JSCFunctionMagic *fun
                                            int length, JSCFunctionEnum cproto, int magic)
 {
     /* Used to squelch a -Wcast-function-type warning. */
-    JSCFunctionType ft = { .generic_magic = func };
+    JSCFunctionType ft;
+    ft.generic_magic = func;
     return JS_NewCFunction2(ctx, ft.generic, name, length, cproto, magic);
 }
 JS_EXTERN void JS_SetConstructor(JSContext *ctx, JSValue func_obj,