replace VLAs with standard C++

src/core/helpers.cpp

@@ -495,11 +495,11 @@ std::string Helpers::data_to_hex(const uint8_t * data, const uint8_t length) {
         return "<empty>";
     }
 
-    char str[length * 3];
+    std::vector<char> str(length * 3);
-    memset(str, 0, sizeof(str));
+    memset(str.data(), 0, str.size());
 
     char   buffer[4];
-    char * p = &str[0];
+    char * p = str.data();
     for (uint8_t i = 0; i < length; i++) {
         Helpers::hextoa(buffer, data[i]);
         *p++ = buffer[0];
@@ -508,7 +508,7 @@ std::string Helpers::data_to_hex(const uint8_t * data, const uint8_t length) {
     }
     *--p = '\0'; // null terminate just in case, loosing the trailing space
 
-    return std::string(str);
+    return std::string(str.data());
 }
 
 // takes a hex string and convert it to an unsigned 32bit number (max 8 hex digits)

src/core/mqtt.cpp

@@ -222,9 +222,11 @@ void Mqtt::on_message(const char * topic, const uint8_t * payload, size_t len) {
     // the payload is not terminated
     // convert payload to a null-terminated char string
     // see https://www.emelis.net/espMqttClient/#code-samples
-    char message[len + 1];
+    // fix variable-length arrays (VLAs) "char message[len + 1]" as they are not standard C++; they're a Clang/GCC extension.
-    memcpy(message, payload, len);
+    std::vector<char> message_buffer(len + 1);
-    message[len] = '\0';
+    memcpy(message_buffer.data(), payload, len);
+    message_buffer[len] = '\0';
+    char * message      = message_buffer.data();
 
 #if defined(EMSESP_DEBUG)
     if (len) {