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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
fixes this.


Thanks Christian
for the investigation.

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

There are several c++17  bugs around

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


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