Re: GCC 7.3.0 -std=gnu++17 failed to getline() from std::ifstream

[Ross Smith <ross.smith@otoy.com>]

On 2018-06-13 05:11, Christian Franke wrote:
> Ivan Shynkarenka wrote:
>> I use x64 bit Cygwin and it failed in my home, work and Appveyor.  I add
>> cygcheck.out with my environment.
>>
>> I'm sorry about misspell prefix space in my prev example. Please try the
>> following one:
>>
>> #include <fstream>
>> #include <iostream>
>>
>> int main(int argc, char** argv)
>> {
>>      std::string line;
>>      std::ifstream stream("test.cpp");
>>      while (getline(stream, line))
>>          std::cout << line << std::endl;
>>      return 0;
>> }
>>
>> g++ -std=gnu++17 test.cpp
> 
> Could reproduce this with 32 and 64 bit Cygwin g++ 7.3.0
> 
> A comparison of preprocessor (-E) outputs shows that the "extern 
> template" declarations for getline() are only visible for C++ <= 14. 
> These are guarded by "__cplusplus <= 1402" in basic_string.tcc. This 
> should tell the compiler to generate new code for getline() if C++17 is 
> enabled instead of calling the (now incompatible) function in 
> cygstdc++-6.dll.
> 
> A comparison of assembly (-S) outputs shows that this does not work: If 
> C++17 is enabled, the compiler correctly generates local code for 
> getline(istream &, string &) but this code calls an external 
> getline(istream &, string &, char). Then the linker generates a call to 
> this getline() in cygstdc++-6.dll.
> 
> This is because there is a bogus prototype specialization for 
> getline(istream &, string &, char) in basic_string.h but no 
> corresponding implementation in basic_string.tcc. This has apparently an 
> equivalent effect as 'extern template'.
> 
> The attached patch for
>    /usr/lib/gcc/*-pc-cygwin/7.3.0/include/c++/bits/basic_string.h
> fixes this.
> 
> Christian

Thank you! I can confirm that the patch fixes this.

Ross Smith




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