[Libav-user] g++ 4.7.2 fails to compile av_err2str

"René J.V. Bertin" rjvbertin at gmail.com
Wed Jan 16 16:09:21 CET 2013

On Jan 16, 2013, at 09:45, Alex Cohn wrote:

> The difference is that compound literal is resolved at compile time,
> so using it inside a loop does not produce extra instances.

Evidently, as long as this is not a decision that's left to the compiler.

> Here is my C++ way of defining av_err2str:
>    #ifdef  __cplusplus
>    static const std::string av_make_error_string(int errnum)
>    {
>        char errbuf[AV_ERROR_MAX_STRING_SIZE];
>        av_strerror(errnum, errbuf, AV_ERROR_MAX_STRING_SIZE);
>        return (std::string)errbuf;
>    }
>    #undef av_err2str
>    #define av_err2str(errnum) av_make_error_string(errnum).c_str()
>    #endif // __cplusplus

Almost exactly as I also would have done it, or else using a specific new class containing a modifiable buffer of size AV_ERROR_MAX_STRING_SIZE to avoid the local variable and copying the string.

But I have a doubt (I'm far from a C++ expert) ... what's the scope of the object returned as return (std::string)errbuf? AFAIK C++ doesn't have a retain/release mechanism like ObjC has, so is it the compiler that keeps track of when to deconstruct the returned string object? In C, returning a local string variable comes without warranty that the contents will still be available when the caller wants to access them!


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