Splitting C++ Strings Onto Multiple Lines (Code Syntax, Not Parsing)

Question

Not to be confused with how to split a string parsing wise, e.g.:
Split a string in C++?

I am a bit confused as to how to split a string onto multiple lines in c++.

This sounds like a simple question, but take the following example:

#include <iostream>
#include <string>
main() {
  //Gives error
  std::string my_val ="Hello world, this is an overly long string to have" +
    " on just one line";
  std::cout << "My Val is : " << my_val << std::endl;

  //Gives error
  std::string my_val ="Hello world, this is an overly long string to have" &
    " on just one line";  
  std::cout << "My Val is : " << my_val << std::endl;
}

I realize that I could use the std::string append() method, but I was wondering if there was any shorter/more elegant (e.g. more pythonlike, though obviously triple quotes etc. aren't supported in c++) way to break strings in c++ onto multiple lines for sake of readability.

One place where this would be particularly desirable is when you're passing long string literals to a function (for example a sentence).

Here is an interesting tidbit: The C++ lexer doesn't actually care about how many quotes you place before and after a string with only two exceptions. The number of quotes you use must be odd and they must match on either side. `""""" This is a valid string and will be parsed """""`. However, there are no special properties given to these strings, they simply behave like single quotes. — Thomas Anthony, Jun 18 '12 at 16:15
Interesting thanks for sharing that... is there any useful purpose for that? You might be able to use that as a subtle tag for different groups of strings in a code for external parsing by perl/bash/python scripts. That's about all I can think of for now. :) — Jason R. Mick, Aug 24 '15 at 02:22
@ThomasAnthony That just happens because it will treat the quotes around the end as a bunch of empty strings and concatenate them together - it's not a feature, it's standard C/C++ behaviour — texasflood, Dec 04 '15 at 10:57

Eclipse · Accepted Answer · 2010-10-04T21:48:03.373

150

Don't put anything between the strings. Part of the C++ lexing stage is to combine adjacent string literals (even over newlines and comments) into a single literal.

#include <iostream>
#include <string>
main() {
  std::string my_val ="Hello world, this is an overly long string to have" 
    " on just one line";
  std::cout << "My Val is : " << my_val << std::endl;
}

Note that if you want a newline in the literal, you will have to add that yourself:

#include <iostream>
#include <string>
main() {
  std::string my_val ="This string gets displayed over\n" 
    "two lines when sent to cout.";
  std::cout << "My Val is : " << my_val << std::endl;
}

If you are wanting to mix a #defined integer constant into the literal, you'll have to use some macros:

#include <iostream>
using namespace std;

#define TWO 2
#define XSTRINGIFY(s) #s
#define STRINGIFY(s) XSTRINGIFY(s)

int main(int argc, char* argv[])
{
    std::cout << "abc"   // Outputs "abc2DEF"
        STRINGIFY(TWO)
        "DEF" << endl;
    std::cout << "abc"   // Outputs "abcTWODEF"
        XSTRINGIFY(TWO) 
        "DEF" << endl;
}

There's some weirdness in there due to the way the stringify processor operator works, so you need two levels of macro to get the actual value of TWO to be made into a string literal.

edited Oct 04 '10 at 21:48

answered Oct 04 '10 at 21:15

Eclipse

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Can you mix in other items like an int constant? If not how do I add these? – Jason R. Mick Oct 04 '10 at 21:22
@Jason: What do you mean by mixing in an int constant? – Eclipse Oct 04 '10 at 21:27
my_function(arg_1, arg_2, "hello world this string is " TWO "lines long!" ); – Jason R. Mick Oct 04 '10 at 21:29
with a line break between the `TWO` and the `"lines long"` – Jason R. Mick Oct 04 '10 at 21:31
4

Combining string literals is actual part of the lexical phase (not the pre-processor), as string splitting like this is defined as part of the language. – Martin York Oct 04 '10 at 21:46
@Eclipse... this was too long to express clearly in a comment, so I broke it into a separate question... see my latest question, and take a crack at it, if you can. – Jason R. Mick Oct 04 '10 at 23:40
1

@Jason, from your follow-up question, it looks like you are needing something like the `STRINGIFY` macro above. – Eclipse Oct 05 '10 at 00:48

score 11 · Answer 2 · answered Oct 04 '10 at 21:15

11

Are they both literals? Separating two string literals with whitespace is the same as concatenation: "abc" "123" is the same as "abc123". This applies to straight C as well as C++.

answered Oct 04 '10 at 21:15

Matt K

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score 8 · Answer 3 · answered Oct 04 '10 at 22:38

8

I don't know if it is an extension in GCC or if it is standard, but it appears you can continue a string literal by ending the line with a backslash (just as most types of lines can be extended in this manor in C++, e.g. a macro spanning multiple lines).

#include <iostream>
#include <string>

int main ()
{
    std::string str = "hello world\
    this seems to work";

    std::cout << str;
    return 0;
}

answered Oct 04 '10 at 22:38

rmeador

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That literal syntax contains quite a few blanks between `world` and `this` – SingleNegationElimination Oct 04 '10 at 22:52
1

@TokenMacGuy: indeed it does, I hadn't noticed that. Easy enough to fix if you move the second (and subsequent) lines all the way to the left. But that would really mess with the appearance of your indenting. – rmeador Oct 05 '10 at 15:29
Yea that was my initial approach, but I abandoned it due to the indentation/spacing issues @SingleNegationElimination outlines. Good to note though. – Jason R. Mick Aug 24 '15 at 02:24

Splitting C++ Strings Onto Multiple Lines (Code Syntax, Not Parsing)

3 Answers3

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