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Re: [patch] Implement ISO/IEC TS 18822 C++ File system TS

2015-05-01 20:22 GMT+02:00 Jonathan Wakely <>:
> On 01/05/15 19:03 +0200, Daniel Krügler wrote:
>> b/libstdc++-v3/src/filesystem/
>> - path::compare(const path& p) const noexcept:
>> Shouldn't the implementation of this noexcept function not try to
>> create copies of path objects? Couldn't _Cmpt just hold references to
>> _M_pathname?
> All your other comments are definitely correct and I'll make the
> relevant fixes soon, but I'm not sure what you mean here, could you
> clarify?

My remembrance of the difference in noexcept qualifications of

int  compare(const path& p) const noexcept;
int  compare(const string_type& s) const;
int  compare(const value_type* s) const;

is that the latter are allowed to allocate memory to be implemented as
the standard writes the corresponding functions for std::basic_string:

int compare(const basic_string& str) const noexcept;
int compare(const charT* s) const;

Returns: compare(basic_string(s)).

But if I read your implementation of path::compare(const path& p)
correctly it *also* may allocate memory by copying _M_pathname into a
_Cmpt object. I was wondering whether for this comparison there exists
real need to *copy* _M_pathname (potentially exceeding the memory
limits). Wouldn't it be possible to define a _CmptRef type that for
the purpose of implementing compare(const path& p) just refers to
references of the _M_pathname and therefore doesn't need to allocate
any dynamic storage? (I should have spoken of a new type _CmptRef and
not of your existing _Cmpt).

> I think it would be possible to implement a path like:

Actually I was not (yet) suggesting to change your API of path, I was
only wondering whether given the existing ABI compare(const path& p)
could be implementable without need of additional dynamic memory. This
was more a kind of louder thinking, I have not fully thought whether
this not possible without changing your currently suggested ABI.

- Daniel

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