2 
char webpage[] PROGMEM = R"=====( 
<html>
  <head>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/svg.js/2.6.6/svg.min.js"></script>
  </head>
  <body>
  </body>
</html>
)=====";

I found a problem when using the raw string literal in Arduino IDE. My code cannot compile if i have http:// link inside the string.

I found that it should be related to how Arduino IDE thought the string after // are comments and during compile time those string are ignored and my codes break.

It looks like the arduino ide's compiler removes all //comment in the code first, before it try to compile the raw string literal and then cause the issue.

As a test I try to debug it by changing my code to https:/-/ the program can be compile as now there is no // in the raw string literal

Is there any workaround?

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0

3 Answers 3

Reset to default
3

Escape the two quote marks inside the string using a backslash (literal quotes).

Without the escape characters, the URL is "outside" of a string because the quote marks are string delimiters.

char webpage[] PROGMEM = R"=====( 
<html>
  <head>
    <script src=\"https://cdnjs.cloudflare.com/ajax/libs/svg.js/2.6.6/svg.min.js\"></script>
  </head>
  <body>
  </body>
</html>
)=====";
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4
  • thanks. interesting. after escaping the " it can complie but the link doesn't work anymore because in the html the src link cannot point to the link location anymore, as it got the extra \ escape character before "
    – Ho Chung Law
    Oct 26, 2018 at 6:28
  • Escaping characters are not left into the end result, you have some other issue, for example you can't link to external content when serving directly from an ESP8266/ESP32 because it doesn't have internet connection that could enable your device to download the resources.
    – Avamander
    Oct 26, 2018 at 11:14
  • This is just a simplified snippet of the code. My original program will serve a javascript page. I will post my full code later.
    – Ho Chung Law
    Oct 27, 2018 at 4:16
  • For c11 raw string literal, the end result will contain the escape characters. Meaning \" in the raw string literal will be still display as \"
    – Ho Chung Law
    Oct 27, 2018 at 7:49
3 
char webpage[] PROGMEM = R"=="==( 
<html>
  <head>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/svg.js/2.6.6/svg.min.js"></script>
  </head>
  <body>
  </body>
</html>
)=="==";

inspired by answer provided by @jsotola if i used the syntax like this R"=="==()=="==" the link will become inside the quote. and hence the ide's complier regards it as a string and won't think // is comment and now the code can complie and the link works.

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0

The C & C++ compilers don't look for language syntax inside quoted strings (except for character-escapes such as '\n'). That is the point of quoting the string.

This program just compiled Ok for me:

const char webpage[] PROGMEM = R"=====( 
<html>
  <head>
    <script src=\"https://cdnjs.cloudflare.com/ajax/libs/svg.js/2.6.6/svg.min.js\"></script>
  </head>
  <body>
  </body>
</html>
)=====";

void setup() {
}

void loop() {
}

'//' within the string did not cause any problem. Your example did get an error message that the variable 'webpage' had to be const to put it in Flash memory so I added the 'const' keyword, but that is the only change I made.

If you do that, and your compiler is still complaining, there is some other issue than the '//'.

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