To reproduce: <div class="highlight highlight-source-c++ notranslate position-rela

<a class="user-mention notranslate" data-hovercard-type="user" data-hovercard-url="/us

I have further simplified the example with <a href="https://gist.github.com/am11/5341b

<a class="user-mention notranslate" data-hovercard-type="user" data-hovercard-url="/us

Redirected stdin C++ stream behaves weirdly with string literals on windows about criterion HOT 10 CLOSED

snaipe commented on June 9, 2024

Redirected stdin C++ stream behaves weirdly with string literals on windows

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Comments (10)

am11 commented on June 9, 2024

@Snaipe, can you please make a more self-contained example (with main instantiating a &ofstream and then expected/actual outputs)? I will pass it along to some concerned folks. :)

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Snaipe commented on June 9, 2024

@am11 I managed to come up with a self-contained example after a bit of tweaking around.

The issue arises only when:

the stream class is inherited.
the variable is typed as a reference of the inherited type (using a reference to std::fstream, for instance, works).

It seems to be a problem with the overload resolution picking up operator<< for const void * before the one for const char *.

GCC & Clang seem to both handle the resolution gracefuly.

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am11 commented on June 9, 2024

@Snaipe, thanks! On compiling with g++ 4.9, I am getting:

sh-4.3$ g++ --std=c++0x -o main *.cpp                                                                                                                                              

main.cpp: In instantiation of 'my_basic_fstream<CharT>::my_basic_fstream(FILE*) [with CharT = char; FILE = _IO_FILE]':                                                             

main.cpp:21:13:   required from 'static my_basic_fstream<CharT>& get_file_stream_<CharT>::call(FILE*) [with CharT = char; FILE = _IO_FILE]'                                        

main.cpp:28:36:   required from here                                                                                                                                               

main.cpp:11:64: error: no matching function for call to 'std::basic_fstream<char>::basic_fstream(FILE*&)'                                                                          

     my_basic_fstream(std::FILE *f) : std::basic_fstream<char>(f) {}                                                                                                               

                                                                ^                                                                                                                  

main.cpp:11:64: note: candidates are:                                                                                                                                              

In file included from main.cpp:3:0:                                                                                                                                                

/usr/include/c++/4.9.2/fstream:834:7: note: std::basic_fstream<_CharT, _Traits>::basic_fstream(const string&, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_tra

its<char>; std::string = std::basic_string<char>; std::ios_base::openmode = std::_Ios_Openmode]                                                                                    

       basic_fstream(const std::string& __s,                                                                                                                                       

       ^                                                                                                                                                                           

/usr/include/c++/4.9.2/fstream:834:7: note:   no known conversion for argument 1 from 'FILE* {aka _IO_FILE*}' to 'const string& {aka const std::basic_string<char>&}'              

/usr/include/c++/4.9.2/fstream:819:7: note: std::basic_fstream<_CharT, _Traits>::basic_fstream(const char*, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_trait

s<char>; std::ios_base::openmode = std::_Ios_Openmode]                                                                                                                             

       basic_fstream(const char* __s,                                                                                                                                              

       ^                                                                                                                                                                           

/usr/include/c++/4.9.2/fstream:819:7: note:   no known conversion for argument 1 from 'FILE* {aka _IO_FILE*}' to 'const char*'                                                     

/usr/include/c++/4.9.2/fstream:806:7: note: std::basic_fstream<_CharT, _Traits>::basic_fstream() [with _CharT = char; _Traits = std::char_traits<char>]                            

       basic_fstream()                                                                                                                                                             

       ^                                                                                                                                                                           

/usr/include/c++/4.9.2/fstream:806:7: note:   candidate expects 0 arguments, 1 provided

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am11 commented on June 9, 2024

I have further simplified the example with https://gist.github.com/am11/5341b87ce9eb6bdf2d15; added some comments. Removing one (apparently) unrelated and unused code line makes it compile and pass with VCR. But Clang and GCC failed due to same issue (that they give with the original example): missing function signature std::basic_fstream which takes std::File*.

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Snaipe commented on June 9, 2024

@am11 the sample is using a MSVC-specific constructor (the one taking a FILE*), I did not make the GCC/Clang specific version because it did not happen on both of them.

I added a version which is GCC/Clang compatible, but it's not that interesting to be honest.

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am11 commented on June 9, 2024

@Snaipe, I agree its looks like spaghetti now. 😜
Nonetheless, I will proceed with reporting to VC team and cross-reference the issue here. FWIW, we reported some bugs after VS2015 RTM (released 29.07.2015) and they have fixed for VS2015 Update 1 (currently unreleased). For some reason their webcompiler site is down: http://webcompiler.cloudapp.net/, otherwise we could verify this one right away.
Thank you for this effort! 👍

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am11 commented on June 9, 2024

x-ref: https://connect.microsoft.com/VisualStudio/feedback/details/1816111.

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Snaipe commented on June 9, 2024

@am11 I just reread the snippet you linked, but I think that the solution you provide to make it pass is a bit unfortunate: *unused_stream actively invokes undefined behaviour (you dereference a null smart pointer), and, well, the apparent "good" behaviour could be the result of an optimisation based on the fact that *unused_stream is invalid.

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am11 commented on June 9, 2024

@Snaipe, I have received a reply (via private email) from VC team member (Stephan T. Lavavej aka STL):

I've reduced this and I believe it is a compiler bug. Here's a library-free repro:

C:\Temp>type meow.cpp
namespace Std {

    template <class Elem> struct CharTraits { };

    template <class Elem, class Traits = CharTraits<Elem>> struct BasicOstream {
        void operator<<(const void *) = delete;
    };

    template <class Traits> void operator<<(BasicOstream<char, Traits>&, const char *) = delete;
}

template <typename CharT> struct MyOstream : Std::BasicOstream<char> { };

MyOstream<char> * stream;

#ifdef AVOID_TEMPLATE
    MyOstream<char>& call() {
#else
    template <typename RandomT = double> MyOstream<char>& call() {
#endif
    stream = new MyOstream<char>();
    return *stream;
}

int main() {
    MyOstream<char>& f = call();
    f << "Foo";
}
C:\Temp>cl /EHsc /nologo /W4 meow.cpp

meow.cpp

meow.cpp(26): error C2280: 'void Std::BasicOstream<char,Std::CharTraits>::operator <<(const void *)': attempting t

o reference a deleted function
    with

    [

        Elem=char

    ]
meow.cpp(5): note: see declaration of 'Std::BasicOstream<char,Std::CharTraits>::operator <<'
    with

    [

        Elem=char

    ]
C:\Temp>cl /EHsc /nologo /W4 /DAVOID_TEMPLATE meow.cpp

meow.cpp

meow.cpp(26): error C2280: 'void Std::operator <<Std::CharTraits(Std::BasicOstream<Elem,Std::CharTraits> &

,const char *)': attempting to reference a deleted function
    with

    [

        Elem=char
meow.cpp(8): note: see declaration of 'Std::operator <<'

AVOID_TEMPLATE shouldn't affect the result of overload resolution, but it does. I'll pass this along to the compiler team.

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Snaipe commented on June 9, 2024

Awesome.

In the meantime, I guess I can remove the template on call (we use CharT = char anyways) since it's affecting resolution.

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Redirected stdin C++ stream behaves weirdly with string literals on windows about criterion HOT 10 CLOSED

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