避免 C++11 原始字符串文字中的第一个换行符？

Question

The raw string literals in C++11 are very nice, except that the obvious way to format them leads to a redundant newline \n as the first character.

Consider this example:

    some_code();
    std::string text = R"(
This is the first line.
This is the second line.
This is the third line.
)";
    more_code();

The obvious workaround seems so ugly:

    some_code();
    std::string text = R"(This is the first line.
This is the second line.
This is the third line.
)";
    more_code();

Has anyone found an elegant solution to this?

Answer 1

You can get a pointer to the 2nd character - skipping the leading newline - by adding 1 to the const char* to which the string literal is automatically converted:

    some_code();
    std::string text = 1 + R"(
This is the first line.
This is the second line.
This is the third line.
)";
    more_code();

---

IMHO, the above is flawed in breaking with the indentation of the surrounding code. Some languages provide a built-in or library function that does something like:

• removes an empty leading line, and
• looks at the indentation of the second line and removes the same amount of indentation from all further lines

That allows usage like:

some_code();
std::string text = unindent(R"(
    This is the first line.
    This is the second line.
    This is the third line.
    )");
more_code();

Writing such a function is relatively simple...

std::string unindent(const char* p)
{
    std::string result;
    if (*p == '\n') ++p;
    const char* p_leading = p;
    while (std::isspace(*p) && *p != '\n')
        ++p;
    size_t leading_len = p - p_leading;
    while (*p)
    {
        result += *p;
        if (*p++ == '\n')
        {
            for (size_t i = 0; i < leading_len; ++i)
                if (p[i] != p_leading[i])
                    goto dont_skip_leading;
            p += leading_len;
        }
      dont_skip_leading: ;
    }
    return result;
}

(The slightly weird p_leading[i] approach is intended to make life for people who use tabs and spaces no harder than they make it for themselves ;-P, as long as the lines start with the same sequence.)

Answer 2

This is probably not what you want, but just in case, you should be aware of automatic string literal concatenation:

    std::string text =
"This is the first line.\n"
"This is the second line.\n"
"This is the third line.\n";

Answer 3

I recommend @Brian's answer, especially if you only need to have few lines of text, or that which you can handle with your text editor-fu. I have an alternative if that isn't the case.

    std::string text =
"\
This is the first line." R"(
This is the second line.
This is the third line.)";

Live example

Raw string literals can still concatenate with "normal" string literals, as shown in the code. The "\\ at the start is meant to "eliminate" the " character from the first line, putting it in a line of its own instead.

Still, if I were to decide, I would put such lotsa-text into a separate file and load it at runtime. No pressure to you though :-).

^{Also, that is one of the uglier code I've written these days.}

Answer 4

The closest I can see is:

std::string text = ""
R"(This is the first line.
This is the second line.
This is the third line.
)";

It would be a bit nicer if a whitespace was allowed in the delimiter sequence. Give or take the indentation:

std::string text = R"
    (This is the first line.
This is the second line.
This is the third line.
)
    ";

My preprocessor will let you off with a warning about this, but unfortunately it's a bit useless. Clang and GCC get thrown off completely.

Answer 5

The accepted answer produces the warning cppcoreguidelines-pro-bounds-constant-array-index from clang-tidy . See Pro.bounds: Bounds safety profile for details.

If you don't have std::span but you're at least compiling with C++17 consider:

constexpr auto text = std::string_view(R"(
This is the first line.
This is the second line.
This is the third line.
)").substr(1);

The main advantages are readability (IMHO) and that you can turn on that clang-tidy warning in the rest of your code.

Using gcc if someone does inadvertently reduce the raw string to an empty string you get a compiler error ( demo ) with this approach, while the accepted approach either produces nothing ( demo ) or depending on your compiler settings an "outside bounds of constant string" warning.

Answer 6

Yep, that is annoying. Perhaps there should be raw literals ( R"PREFIX(" ) and multiline raw literals ( M"PREFIX ).

I came up with this alternative which almost describes itself:

#include<iterator> // std::next
...
{
    ...
    ...
    std::string atoms_text = 
std::next/*_line*/(R"XYZ(
  O123        12.4830720891       13.1055820441        9.5288258996
  O123        13.1055820441       13.1055820441        9.5288258996
)XYZ");
    assert( atoms_text[0] != '\n' );
    ...
}

Limitations:

1. If the raw literal is empty it will generate an invalid string. But that should be obvious to spot.
2. If the raw literal doesn't start with a new line it will eat the first character instead.
3. std::next is constexpr only from C++17, you then can use 1+(char const*)R"XYZ(" but it is not as clear and might produce warning.

constexpr auto atom_text = 1 + (R"XYZ(
  O123        12.4830720891       13.1055820441        9.5288258996
  O123        13.1055820441       13.1055820441        9.5288258996
)XYZ");

Also, no warranties ;) . After all, I don't know if it is legal to do arithmetic with pointers to static data.

---

Another advantage of the + 1 approach is that it can be put at the end:

constexpr auto atom_text = R"XYZ(
  O123        12.4830720891       13.1055820441        9.5288258996
  O123        13.1055820441       13.1055820441        9.5288258996
)XYZ" + 1;

---

Possibilities are endless:

constexpr auto atom_text = &R"XYZ(
  O123        12.4830720891       13.1055820441        9.5288258996
  O123        13.1055820441       13.1055820441        9.5288258996
)XYZ"[1];

constexpr auto atom_text = &1[R"XYZ(
  O123        12.4830720891       13.1055820441        9.5288258996
  O123        13.1055820441       13.1055820441        9.5288258996
)XYZ"];

Answer 7

I had the very same problem and I think the following solution is the best of all the above. I hope it'll be helpful for you, too (see example in the comment):

/**
 * Strips a multi-line string's indentation prefix.
 *
 * Example:
 * \code
 *   string s = R"(|line one
 *                 |line two
 *                 |line three
 *                 |)"_multiline;
 *   std::cout << s;
 * \endcode
 *
 * This prints three lines: @c "line one\nline two\nline three\n"
 *
 * @author Christian Parpart <christian@parpart.family>
 */

inline std::string operator ""_multiline(const char* text, unsigned long size) {
  if (!*text)
    return {};

  enum class State {
    LineData,
    SkipUntilPrefix,
  };

  constexpr char LF = '\n';
  State state = State::LineData;
  std::stringstream sstr;
  char sep = *text++;

  while (*text) {
    switch (state) {
      case State::LineData: {
        if (*text == LF) {
          state = State::SkipUntilPrefix;
          sstr << *text++;
        } else {
          sstr << *text++;
        }
        break;
      }
      case State::SkipUntilPrefix: {
        if (*text == sep) {
          state = State::LineData;
          text++;
        } else {
          text++;
        }
        break;
      }
    }
  }

  return sstr.str();
}

Answer 8

With C++20 this can now be implemented fully at compile-time by using a string literal operator template .

That has a few key benefits:

• Only the unindented string will be stored in the resulting binary.
• No allocations, zero runtime overhead
• The resulting value will be a reference to a character array ( const char (&)[N] ) - like normal character literals in C++; so no std::array shenanigans and lifetime issues.

Usage Example: godbolt

std::cout << R"(
     a
    b
     c
    d
)"_M << std::endl;
/* Will print the following:
 a
b
 c
d
*/

// The type of R"(...)"_M is const char (&)[N],
// so it can be used like a normal string literal:
std::cout << std::size(R"(asdf)"_M) << std::endl;
// (will print 5)
constexpr std::string_view str = R"(
  foo
  bar
)"_M;
// str == "foo\nbar"

// also works with wchar_t, char8_t, char16_t and char32_t literals:
std::wcout << LR"(
  foo
  bar
)"_M;
std::wcout << std::endl;

---

Normally it's not possible to pass string literals as template arguments, eg:

template<const char* str>
void foo();

// ill-formed
foo<"bar">();

But with C++20 we can now have class-type template arguments, and those could be constant-initialized from a string literal.

That in combination with the new string literal operator templates makes it possible to get the entire string literal as a template parameter:

template<class _char_type, std::size_t size>
struct string_wrapper {
    using char_type = _char_type;

    consteval string_wrapper(const char_type (&arr)[size]) {
        std::ranges::copy(arr, str);
    }

    char_type str[size];
};

template<string_wrapper str>
consteval decltype(auto) operator"" _M() {
    /*...*/
}

// R"(foobar)"_M
// would now result in the following code:
// operator"" _M<string_wrapper<char, 7>{"foobar"}>()

Having both the length and the individual characters as constant expressions enables us now to compute the required size for the unindented string fully at compile-time and storing the resulting string in another template parameter (so that we merely need to return a reference to the final string value):

// unindents the individual lines of a raw string literal
// e.g. unindent_string("  \n  a\n  b\n  c\n") -> "a\nb\nc"
template<class char_type>
consteval std::vector<char_type> unindent_string(string_view<char_type> str) {
    /* ... */
}

// returns the size required for the unindented string
template<class char_type>
consteval std::size_t unindent_string_size(string_view<char_type> str) {
    /* ... */
}

// used for sneakily creating and storing
// the unindented string in a template parameter.
template<string_wrapper sw>
struct unindented_string_wrapper {
    using char_type = typename decltype(sw)::char_type;
    static constexpr std::size_t buffer_size = unindent_string_size<char_type>(sw.str);
    using array_ref = const char_type (&)[buffer_size];

    consteval unindented_string_wrapper(int) {
        auto newstr = unindent_string<char_type>(sw.str);
        std::ranges::copy(newstr, buffer);
    }

    consteval array_ref get() const {
        return buffer;
    }

    char_type buffer[buffer_size];
};

// uses a defaulted template argument that depends on the str
// to initialize the unindented string within a template parameter.
// this enables us to return a reference to the unindented string.
template<string_wrapper str, unindented_string_wrapper<str> unindented = 0>
consteval decltype(auto) do_unindent() {
    return unindented.get();
}

// the actual user-defined string literal operator
template<string_wrapper str>
consteval decltype(auto) operator"" _M() {
    return do_unindent<str>();
}

---

Full Code: godbolt

#include <algorithm>
#include <string_view>
#include <vector>
#include <ranges>

namespace multiline_raw_string {
    template<class char_type>
    using string_view = std::basic_string_view<char_type>;

    // characters that are considered space
    // we need this because std::isspace is not constexpr
    template<class char_type>
    constexpr string_view<char_type> space_chars = std::declval<string_view<char_type>>();
    template<>
    constexpr string_view<char> space_chars<char> = " \f\n\r\t\v";
    template<>
    constexpr string_view<wchar_t> space_chars<wchar_t> = L" \f\n\r\t\v";
    template<>
    constexpr string_view<char8_t> space_chars<char8_t> = u8" \f\n\r\t\v";
    template<>
    constexpr string_view<char16_t> space_chars<char16_t> = u" \f\n\r\t\v";
    template<>
    constexpr string_view<char32_t> space_chars<char32_t> = U" \f\n\r\t\v";
    
    
    // list of all potential line endings that could be encountered
    template<class char_type>
    constexpr string_view<char_type> potential_line_endings[] = std::declval<string_view<char_type>[]>();
    template<>
    constexpr string_view<char> potential_line_endings<char>[] = {
        "\r\n",
        "\r",
        "\n"
    };
    template<>
    constexpr string_view<wchar_t> potential_line_endings<wchar_t>[] = {
        L"\r\n",
        L"\r",
        L"\n"
    };
    template<>
    constexpr string_view<char8_t> potential_line_endings<char8_t>[] = {
        u8"\r\n",
        u8"\r",
        u8"\n"
    };
    template<>
    constexpr string_view<char16_t> potential_line_endings<char16_t>[] = {
        u"\r\n",
        u"\r",
        u"\n"
    };
    template<>
    constexpr string_view<char32_t> potential_line_endings<char32_t>[] = {
        U"\r\n",
        U"\r",
        U"\n"
    };

    // null-terminator for the different character types
    template<class char_type>
    constexpr char_type null_char = std::declval<char_type>();
    template<>
    constexpr char null_char<char> = '\0';
    template<>
    constexpr wchar_t null_char<wchar_t> = L'\0';
    template<>
    constexpr char8_t null_char<char8_t> = u8'\0';
    template<>
    constexpr char16_t null_char<char16_t> = u'\0';
    template<>
    constexpr char32_t null_char<char32_t> = U'\0';

    // detects the line ending used within a string.
    // e.g. detect_line_ending("foo\nbar\nbaz") -> "\n"
    template<class char_type>
    consteval string_view<char_type> detect_line_ending(string_view<char_type> str) {
        return *std::ranges::max_element(
            potential_line_endings<char_type>,
            {},
            [str](string_view<char_type> line_ending) {
                // count the number of lines we would get with line_ending
                auto view = std::views::split(str, line_ending);
                return std::ranges::distance(view);
            }
        );
    }

    // returns a view to the leading sequence of space characters within a string
    // e.g. get_leading_space_sequence(" \t  foo") -> " \t  "
    template<class char_type>
    consteval string_view<char_type> get_leading_space_sequence(string_view<char_type> line) {
        return line.substr(0, line.find_first_not_of(space_chars<char_type>));
    }

    // checks if a line consists purely out of space characters
    // e.g. is_line_empty("    \t") -> true
    //      is_line_empty("   foo") -> false
    template<class char_type>
    consteval bool is_line_empty(string_view<char_type> line) {
        return get_leading_space_sequence(line).size() == line.size();
    }

    // splits a string into individual lines
    // and removes the first & last line if they are empty
    // e.g. split_lines("\na\nb\nc\n", "\n") -> {"a", "b", "c"}
    template<class char_type>
    consteval std::vector<string_view<char_type>> split_lines(
        string_view<char_type> str,
        string_view<char_type> line_ending
    ) {
        std::vector<string_view<char_type>> lines;

        for (auto line : std::views::split(str, line_ending)) {
            lines.emplace_back(line.begin(), line.end());
        }

        // remove first/last lines in case they are completely empty
        if(lines.size() > 1 && is_line_empty(lines[0])) {
            lines.erase(lines.begin());
        }
        if(lines.size() > 1 && is_line_empty(lines[lines.size()-1])) {
            lines.erase(lines.end()-1);
        }

        return lines;
    }

    // determines the longest possible sequence of space characters
    // that we can remove from each line.
    // e.g. determine_common_space_prefix_sequence({" \ta", " foo", " \t\ŧbar"}) -> " "
    template<class char_type>
    consteval string_view<char_type> determine_common_space_prefix_sequence(
        std::vector<string_view<char_type>> const& lines
    ) {
        std::vector<string_view<char_type>> space_sequences = {
            string_view<char_type>{} // empty string
        };

        for(string_view<char_type> line : lines) {
            string_view<char_type> spaces = get_leading_space_sequence(line);
            for(std::size_t len = 1; len <= spaces.size(); len++) {
                space_sequences.emplace_back(spaces.substr(0, len));
            }
   
            // remove duplicates
            std::ranges::sort(space_sequences);
            auto [first, last] = std::ranges::unique(space_sequences);
            space_sequences.erase(first, last);
        }

        // only consider space prefix sequences that apply to all lines
        auto shared_prefixes = std::views::filter(
            space_sequences,
            [&lines](string_view<char_type> prefix) {
                return std::ranges::all_of(
                    lines,
                    [&prefix](string_view<char_type> line) {
                        return line.starts_with(prefix);
                    }
                );
            }
        );

        // select the longest possible space prefix sequence
        return *std::ranges::max_element(
            shared_prefixes,
            {},
            &string_view<char_type>::size
        );
    }

    // unindents the individual lines of a raw string literal
    // e.g. unindent_string("  \n  a\n  b\n  c\n") -> "a\nb\nc"
    template<class char_type>
    consteval std::vector<char_type> unindent_string(string_view<char_type> str) {
        string_view<char_type> line_ending = detect_line_ending(str);
        std::vector<string_view<char_type>> lines = split_lines(str, line_ending);
        string_view<char_type> common_space_sequence = determine_common_space_prefix_sequence(lines);

        std::vector<char_type> new_string;
        bool is_first = true;
        for(auto line : lines) {
            // append newline
            if(is_first) {
                is_first = false;
            } else {
                new_string.insert(new_string.end(), line_ending.begin(), line_ending.end());
            }

            // append unindented line
            auto unindented = line.substr(common_space_sequence.size());
            new_string.insert(new_string.end(), unindented.begin(), unindented.end());
        }

        // add null terminator
        new_string.push_back(null_char<char_type>);

        return new_string;
    }

    // returns the size required for the unindented string
    template<class char_type>
    consteval std::size_t unindent_string_size(string_view<char_type> str) {
        return unindent_string(str).size();
    }

    // simple type that stores a raw string
    // we need this to get around the limitation that string literals
    // are not considered valid non-type template arguments.
    template<class _char_type, std::size_t size>
    struct string_wrapper {
        using char_type = _char_type;

        consteval string_wrapper(const char_type (&arr)[size]) {
            std::ranges::copy(arr, str);
        }

        char_type str[size];
    };

    // used for sneakily creating and storing
    // the unindented string in a template parameter.
    template<string_wrapper sw>
    struct unindented_string_wrapper {
        using char_type = typename decltype(sw)::char_type;
        static constexpr std::size_t buffer_size = unindent_string_size<char_type>(sw.str);
        using array_ref = const char_type (&)[buffer_size];

        consteval unindented_string_wrapper(int) {
            auto newstr = unindent_string<char_type>(sw.str);
            std::ranges::copy(newstr, buffer);
        }

        consteval array_ref get() const {
            return buffer;
        }

        char_type buffer[buffer_size];
    };

    // uses a defaulted template argument that depends on the str
    // to initialize the unindented string within a template parameter.
    // this enables us to return a reference to the unindented string.
    template<string_wrapper str, unindented_string_wrapper<str> unindented = 0>
    consteval decltype(auto) do_unindent() {
        return unindented.get();
    }

    // the actual user-defined string literal operator
    template<string_wrapper str>
    consteval decltype(auto) operator"" _M() {
        return do_unindent<str>();
    }
}

using multiline_raw_string::operator"" _M;