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

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
fixes this.


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

Ross Smith

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