Quoted Strings Library Proposal (Revision 2)

Introduction

Character strings enclosed in quotation marks are an element of numerous common data formats (e.g. XML, CSV), yet C++ standard library stream I/O offers no direct support. Furthermore, standard library stream I/O has a problem with embedded spaces in strings that can trip the unwary. The proposed solution provides direct support for quoted strings, avoids embedded spaces problems and is more efficient than likely user provided solutions.

The proposal is suitable for either C++1y or a standard library Technical Specification (TS). It is a pure addition that will break no existing standard-conforming user code. It is based on a Boost component that has been shipping for several years. The declarations for the proposed functions can go in a new header or an existing header.

The proposed wording below assumes the target is C++1y and places the function declarations in <iomanip>.

Problem description

C++ standard library stream I/O for strings that contain embedded spaces can produce unexpected results. For example,

std::stringstream ss;
std::string original = "foolish me";
std::string round_trip;

ss << original;
ss >> round_trip;

std::cout << original;   // outputs: foolish me
std::cout << round_trip; // outputs: foolish

assert(original == round_trip); // assert will fire

The proposed quoted stream I/O manipulator places delimiters, defaulted to double-quote ("), around strings on output, and strips off the delimiters on input. This ensures strings with embedded white space round-trip as desired. For example,

std::stringstream ss;
std::string original = "foolish me";
std::string round_trip;

ss << quoted(original);
ss >> quoted(round_trip);

std::cout << original;     // outputs: foolish me
std::cout << round_trip;   // outputs: foolish me

assert(original == round_trip); // assert will not fire

If the string contains the delimiter character, on output that character will be preceded by an escape character, default to backslash (\), as will the escape character itself:

std::cout << quoted("She said \"Hi!\"");  // outputs: "She said \"Hi!\""

Revision history

N3654 - Revision 2 (post-Bristol mailing)

• Swap order of const basic_string& and const char* quoted functions in synopsis per Bristol LWG.
• Change charT delim='"' to charT delim=charT('"') per Bristol LWG.
• Ditto for escape. This wasn't asked for, but seems inconsistent to change for delim without also changing for escape.
• Strike unnecessary verbiage from Returns per Bristol LWG.

• Removed unneeded template parameters traits and Allocator from the const char* signature per Bristol LWG.

• Strike incorrect const from extractor signature per Bristol LWG.

• In two Returns:, clarify as indicated below that the stream's char_type and the charT template parameter must be the same type, as requested by the Bristol LWG.

• No change has been made in response the Bristol LWG query "should the operator== actually be the eq from some character traits?" It seems harmless and very clear to leave the operator== spec wording unchanged. But the LWG has the most expertise to answer the question; I'll defer to them if they want a change.

• Remove unnecessary std:: in several places. (Daniel Krügler)

• Add "member type" in two places for improved clarity. (Daniel Krügler)

• Revert a clarity "fix" and add comment in two places indicating the awkward wording comes from the current working paper. (Daniel Krügler)

• Fix the spec for missing case of insertion via the T13 overload. (Nice catch from Alisdair Meredith)

• Fix !in and in >> s typos. (Jonathan Wakely)

N3570 - Revision 1 (pre-Bristol mailing)

• Remove unnecessary backslash from character literals in default arguments.
• Rewrite the proposed wording to follow the standard library's form for iomanip functions. Note that this has the effect of making error handling well-specified, as requested by the LWG at the Portland meeting.
• Editorial cleanup.

N3431 Initial paper (pre-Portland mailing)

Proposed wording

Gray shaded italic text is commentary, and not to be added to the working paper.

Change 27.7.1 Overview [iostream.format.overview], "Header <iomanip> synopsis" as indicated:

namespace std {
// types T1, T2, ... are unspecified implementation types
T1 resetiosflags(ios_base::fmtflags mask);
T2 setiosflags (ios_base::fmtflags mask);
T3 setbase(int base);
template<charT> T4 setfill(charT c);
T5 setprecision(int n);
T6 setw(int n);
template <class moneyT> T7 get_money(moneyT& mon, bool intl = false);
template <class moneyT> T8 put_money(const moneyT& mon, bool intl = false);
template <class charT> T9 get_time(struct tm* tmb, const charT* fmt);
template <class charT> T10 put_time(const struct tm* tmb, const charT* fmt);

template <class charT>
  T11 quoted(const charT* s, charT delim=charT('"'), charT escape=charT('\\'));

template <class charT, class traits, class Allocator>
  T12 quoted(const basic_string<charT, traits, Allocator>& s,
             charT delim=charT('"'), charT escape=charT('\\'));

template <class charT, class traits, class Allocator>
  T13 quoted(basic_string<charT, traits, Allocator>& s,
             charT delim=charT('"'), charT escape=charT('\\'));
}

After 27.7.5 Extended manipulators [ext.manip], add a new sub-section:

27.7.6 Quoted manipulators [quoted.manip]

[Note: Quoted manipulators provide string insertion and extraction of quoted strings (for example, XML and CSV formats). Quoted manipulators are useful in ensuring that the content of a string with embedded spaces remains unchanged if inserted and then extracted via stream I/O. --end note]

template <class charT>
  unspecified quoted(const charT* s, charT delim=charT('"'), charT escape=charT('\\'));
template <class charT, class traits, class Allocator>
  unspecified quoted(const basic_string<charT, traits, Allocator>& s,
                   charT delim=charT('"'), charT escape=charT('\\'));

Returns: An object of unspecified type such that if out is an instance of basic_ostream with member type char_type the same as charT, <charT, traits>, s is an instance of a type convertible to basic_string<charT, traits, Allocator>, or const char*, respectively, and delim and escape are instances of charT, then the expression out << quoted(s, delim, escape) behaves as if it inserts the following characters into out using character inserter function templates ([ostream.inserters.character]), which may throw ios_base::failure ([ios::failure]):

• delim.
• Each character in s. If the character to be output is equal to escape or delim, as determined by operator==, first output escape.
• delim.

The expression out << quoted(s, delim, escape) shall have type basic_ostream<charT, traits>& and value out. This wording is the form for such statements in the current wording paper [std.manip].

template <class charT, class traits, class Allocator>
  unspecified quoted(basic_string<charT, traits, Allocator>& s,
                   charT delim=charT('"'), charT escape=charT('\\'));

Returns: An object of unspecified type such that:

• If in is an instance of basic_istream with member type char_type the same as charT  <charT, traits>, s is an instance of basic_string<charT, traits, Allocator>, and delim and escape are instances of types charT, then the expression in >> quoted(s, delim, escape) behaves as if it extracts the following characters from in using basic_istream::operator>> ([istream::extractors]) which may throw ios_base::failure ([ios::failure]):
   
  • If the first character extracted is equal to delim, as determined by operator==, then:
    • Turn off the skipws flag.
    • s.clear()

    • Until an unescaped delim character is reached or !in, extract characters from in and append them to s, except that if an escape is reached, ignore it and append the next character to s.

    • Discard the final delim character.
    • Restore the skipws flag to its original value.
  • Otherwise, in >> s.
     
• If out is an instance of basic_ostream with member type char_type the same as charT, then the expression out << quoted(s, delim, escape) behaves as specified for the const basic_string<charT, traits, Allocator>& overload of the quoted function.

The expression in >> quoted(s, delim, escape) shall have type basic_istream<charT, traits>& and value in. The expression out << quoted(s, delim, escape) shall have type basic_ostream<charT, traits>& and value out. This wording is the form for such statements in the current wording paper [std.manip].

Acknowledgements

The quoted() stream manipulators emerged from discussions on the Boost developers mailing list. Participants included Beman Dawes, Rob Stewart, Alexander Lamaison, Eric Niebler, Vicente Botet, Andrey Semashev, Phil Richards, and Rob Murray. Eric Niebler's suggestions provided the basis for the name and form of the templates. Thanks to the LWG in Portland and Bristol for additional improvements.