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726. Missing regex_replace() overloads

Section: 28.6.10.4 [re.alg.replace] Status: NAD Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [re.alg.replace].

View all issues with NAD status.

Discussion:

Two overloads of regex_replace() are currently provided:

template <class OutputIterator, class BidirectionalIterator, 
    class traits, class charT> 
  OutputIterator 
  regex_replace(OutputIterator out, 
                BidirectionalIterator first, BidirectionalIterator last, 
                const basic_regex<charT, traits>& e, 
                const basic_string<charT>& fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);
 
template <class traits, class charT> 
  basic_string<charT> 
  regex_replace(const basic_string<charT>& s, 
                const basic_regex<charT, traits>& e, 
                const basic_string<charT>& fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);

1. Overloads taking const charT * are provided for regex_match() and regex_search(), but not regex_replace(). This is inconsistent.

2. The absence of const charT * overloads prevents ordinary-looking code from compiling, such as:

   const string s("kitten");
   const regex r("en");
   cout << regex_replace(s, r, "y") << endl;
   

   The compiler error message will be something like "could not deduce template argument for 'const std::basic_string<_Elem> &' from 'const char[1]'".

   Users expect that anything taking a basic_string<charT> can also take a const charT *. In their own code, when they write a function taking std::string (or std::wstring), they can pass a const char * (or const wchar_t *), thanks to basic_string's implicit constructor. Because the regex algorithms are templated on charT, they can't rely on basic_string's implicit constructor (as the compiler error message indicates, template argument deduction fails first).

   If a user figures out what the compiler error message means, workarounds are available - but they are all verbose. Explicit template arguments could be given to regex_replace(), allowing basic_string's implicit constructor to be invoked - but charT is the last template argument, not the first, so this would be extremely verbose. Therefore, constructing a basic_string from each C string is the simplest workaround.

3. There is an efficiency consideration: constructing basic_strings can impose performance costs that could be avoided by a library implementation taking C strings and dealing with them directly. (Currently, for replacement sources, C strings can be converted into iterator pairs at the cost of verbosity, but for format strings, there is no way to avoid constructing a basic_string.)

[ Sophia Antipolis: ]

We note that Boost already has these overloads. However, the proposed wording is provided only for 28.6.10.4 [re.alg.replace]; wording is needed for the synopsis as well. We also note that this has impact on match_results::format, which may require further overloads.

[ 2009-07 Frankfurt: ]

Daniel to tweak for us.

[ 2009-07-25 Daniel tweaks both this issue and 727⁽ⁱ⁾. ]

This is solved by the proposed resolution of 727⁽ⁱ⁾.

[ 2009-10 Santa Cruz: ]

Leave Open. Though we believe this is solved by the proposed resolution to 727⁽ⁱ⁾.

[ 2010-01-27 Moved to Tentatively NAD after 5 positive votes on c++std-lib. Rationale added below. ]

Rationale:

Solved by 727⁽ⁱ⁾.

Proposed resolution:

Provide additional overloads for regex_replace(): one additional overload of the iterator-based form (taking const charT* fmt), and three additional overloads of the convenience form (one taking const charT* str, another taking const charT* fmt, and the third taking both const charT* str and const charT* fmt). 28.6.10.4 [re.alg.replace]:

template <class OutputIterator, class BidirectionalIterator, 
    class traits, class charT> 
  OutputIterator 
  regex_replace(OutputIterator out, 
                BidirectionalIterator first, BidirectionalIterator last, 
                const basic_regex<charT, traits>& e, 
                const basic_string<charT>& fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);

...

template <class traits, class charT> 
  basic_string<charT> 
  regex_replace(const basic_string<charT>& s, 
                const basic_regex<charT, traits>& e, 
                const basic_string<charT>& fmt, 
                regex_constants::match_flag_type flags = 
                  regex_constants::match_default);