Simple std::regex_search() code won't compile with Apple clang++ -std=c++14

Question

Here is the MCVE:

#include 
#include 

std::string s()
{
    return \"test\";
}

int main()
{
    static const std::regex regex(         


        

Answer 1

This changed between C++11 and C++14. If we go to the cppreference section for std::regex_search we can see that overload that takes an rvalue reference was deleted since C++14:

template< class STraits, class SAlloc,
          class Alloc, class CharT, class Traits > bool regex_search( const std::basic_string&&,
                   std::match_results<
                       typename std::basic_string::const_iterator,
                       Alloc
                   >&,
                   const std::basic_regex&,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default ) = delete;

It was changed due to LWG issue 2329: regex_match()/regex_search() with match_results should forbid temporary strings which says (emphasis mine):

Consider the following code:

const regex r(R"(meow(\d+)\.txt)");
smatch m;
if (regex_match(dir_iter->path().filename().string(), m, r)) {
  DoSomethingWith(m[1]);
}

This occasionally crashes. The problem is that dir_iter->path().filename().string() returns a temporary string, so the match_results contains invalidated iterators into a destroyed temporary string.

It's fine for regex_match/regex_search(str, reg) to accept temporary strings, because they just return bool. However, the overloads taking match_results should forbid temporary strings.

and indeed if we use a non-temporary:

std::string s1 = s() ;

if (std::regex_search(s1, smatch, regex)) {
//...
}

it compiles (see it live) and no longer exhibits undefined behavior.

Interesting to note that gcc/libstdc++ has this overload deleted in C++11 mode as well see it live. Since this is undefined behavior it seems like a good solution.

This issue also pops up in other areas of the library see Visual Studio regex_iterator Bug? which deals with the same issue but with regex_iterator/regex_token_iterator.