diff --git a/lib/preprocessor.cpp b/lib/preprocessor.cpp
index 3d41048fe..e886572c7 100644
--- a/lib/preprocessor.cpp
+++ b/lib/preprocessor.cpp
@@ -1498,6 +1498,8 @@ std::string Preprocessor::getcode(const std::string &filedata, const std::string
 
     // Create a map for the cfg for faster access to defines
     std::map<std::string, std::string> cfgmap(getcfgmap(cfg));
+    if (Path::isCPP(filename) && cfgmap.find("__cplusplus") == cfgmap.end())
+        cfgmap["__cplusplus"] = "1";
 
     std::stack<std::string> filenames;
     filenames.push(filename);
diff --git a/test/testpreprocessor.cpp b/test/testpreprocessor.cpp
index 120615524..7450efa52 100644
--- a/test/testpreprocessor.cpp
+++ b/test/testpreprocessor.cpp
@@ -236,6 +236,7 @@ private:
         TEST_CASE(predefine2);
         TEST_CASE(predefine3);
         TEST_CASE(predefine4);
+        TEST_CASE(predefine5);  // automatically define __cplusplus
 
         // Test Preprocessor::simplifyCondition
         TEST_CASE(simplifyCondition);
@@ -2977,6 +2978,13 @@ private:
         ASSERT_EQUALS("char buf[$123];\n", actual);
     }
 
+    void predefine5() {  // #3737 - automatically define __cplusplus
+        const char code[] = "#ifdef __cplusplus\n123\n#endif";
+        Preprocessor preprocessor(NULL,this);
+        ASSERT_EQUALS("\n\n\n",    preprocessor.getcode(code, "X=123", "test.c"));
+        ASSERT_EQUALS("\n123\n\n", preprocessor.getcode(code, "X=123", "test.cpp"));
+    }
+
     void simplifyCondition() {
         // Ticket #2794
         std::map<std::string, std::string> cfg;