Re: [boost] [system] A non-allocating message() overload

Subject: Re: [boost] [system] A non-allocating message() overload
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2018-09-19 18:38:35

On 9/19/18 8:56 PM, Peter Dimov via Boost wrote:
> One of the objections against <system_error> is that message() needs to 
> allocate memory, because it returns an std::string. This is true even 
> for user-defined categories that can implement it without allocating, 
> with a static message table.
> 
> To address this, I'm thinking of adding the following overload to 
> boost::system::error_category (with the intent of proposing it for 
> std::error_category):
> 
>  Â Â  virtual char const* message( int ev, char* buffer, size_t len ) 
> noexcept;
> 
> This is modeled after the glibc-specific version of strerror_r (and in 
> fact has the exact same signature and behavior). If the implementation 
> has a static character literal corresponding to `ev`, it returns it 
> directly. If not, it composes an error message into the provided buffer, 
> then returns `buffer`.
> 
> Comments?

I like the idea, but what about platforms that don't have strerror_r? 
noexcept means that the new overload cannot be implemented atop the 
standard one (e.g. one can't forward the call to the current 
std::error_category::message).