Sometimes we want to hardcode a multi-line string directly into the C/C++ source code. For example, the usage message is often hardcoded into the source code. Of course we can use line continuation feature of C/C++ by adding a \ sign at the end of a line, but in my opinion, it often makes the code looks rather messy. From early time on, C considers two or more adjacent string literals as one single string, so we can use this feature to make the code more readable. The following is an example.

#include <stdio.h>

const char* poem =
"The Purple Cow\n"
"\n"
"author: Gelett Burgess\n"
"\n"
"I never saw a Purple Cow,\n"
"I never hope to see one,\n"
"But I can tell you, anyhow,\n"
"I'd rather see than be one!";

int main(void) {
  printf("%s\n", poem);
  return 0;
}

One problem with this approach is that we need to put each line in a pair of quotation marks ", and we need to escape specially characters such as quotation mark " itself. Sometimes, we need to embed a json string in the source code of a unittest, then it could turn out to be really annoying, you need to write many \" since json's object/dictionary keys are always strings. A better way to write multi-line string is to take advantage of C/C++ preprocessor's stringification feature and variadic macro feature. We can rewrite the above code using this new approach.

#include <stdio.h>

#define MULTILINE_String(...) #__VA_ARGS__

const char* poem = MULTILINE_String(
The Purple Cow\n
\n
author: Gelett Burgess\n
\n
I never saw a Purple Cow,\n
I never hope to see one,\n
But I can tell you, anyhow,\n
I would rather see than be one!
);

int main(void) {
  printf("%s\n", poem);
  return 0;
}

This form is a lot easier to write, we don't need to write quotation marks for each line. But we still need to write \n for each line if newline character is significant. In some cases, such as json strings, we can safely omit \n. One problem with this approach is that it cannot handle the charater '. I didn't find any solution to include I'd in the string using this method. So I modified I'd to I would in the poem.

In C++11, we have an even better way to write multi-line strings. C++11 introduced raw string literals. You can simply put unescaped string between R"( and )". You can also add your own delimiter. Using this newly introduce feature, we can rewrite the above code as:

#include <cstdio>

const char* poem = R"(
The Purple Cow

author: Gelett Burgess

I never saw a Purple Cow,
I never hope to see one,
But I can tell you, anyhow,
I'd rather see than be one!
)";

int main(void) {
  printf("%s\n", poem);
  return 0;
}