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




On 6/12/2018 7:11 PM, Christian Franke wrote:
Ivan Shynkarenka wrote:
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


Thanks Christian
for the investigation.

It seems an upstream bug so could you report it there ?

There are several c++17  bugs around
https://gcc.gnu.org/bugzilla/buglist.cgi?quicksearch=c%2B%2B17

that is probably the reason why gcc 7.x defaults to c++14

Regards
Marco




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