regex_match 可以访问和不访问已删除 function 的案例

Question

I'm trying to understand why one line of my code compiles one way and another line compiles another way.

I can't pass the direntry.path().wstring() to regex_match directly without getting an "attempting to reference a deleted function" build error.

const std::wstring directoryPath; // function parameter
for (auto& direntry : std::filesystem::directory_iterator(directoryPath))
{
    std::wsmatch fileMatches;
    if (direntry.is_regular_file()
        // Get "attempting to reference a deleted function" on the next line
        && std::regex_match(direntry.path().wstring(), fileMatches, *m_pRegexFile)
        && fileMatches.size() >= 2)
    {
        ...
    }

Same thing compiles OK if I add the filePath variable.

const std::wstring directoryPath; // function parameter
for (auto& direntry : std::filesystem::directory_iterator(directoryPath))
{
    std::wsmatch fileMatches;
    std::wstring filePath = direntry.path().wstring();
    if (direntry.is_regular_file()
        && std::regex_match(filePath, fileMatches, *m_pRegexFile)
        && fileMatches.size() >= 2)
    {
        ...
    }

Meanwhile this works just fine as-is.

std::wregex regexSubdirectory(L"cam.*", std::regex::icase);
for (auto& direntry : std::filesystem::directory_iterator(baseFolder.c_str()))
{
    if (direntry.is_directory()
        && std::regex_match(direntry.path().wstring(), regexSubdirectory))
    {
        ...
    }
}

My code is compiling, but I don't like mysteries. Why does one compile without the variable and the other require it? Best I can tell, one is probably passing const CharT* under the hood, while the other is passing const std::basic_string&. But... I am scratching my head at the definitions ( https://en.cppreference.com/w/cpp/regex/regex_match ) and I think they should both be passing the same thing. Any ideas?

Answer 1

Look at #7 in the list on that cppreference page. The call that takes an rvalue basic_string is marked as deleted.

In your first call, you are passing a temporary string. (rvalue)

In your second call, you are passing an lvalue.

[Later] The reason that that call is deleted is to prevent a programming error. Consider the following bit of code from your example:

std::regex_match(direntry.path().wstring(), fileMatches, *m_pRegexFile);

When this returns, fileMatches contains a bunch of iterators into the string that it matched against. However, the string is a temporary, and once regex_match returns, it will be destroyed. Now accessing any of the matches in fileMatches will be undefined behavior - and probably a crash.