Re: [boost] [conversion] try_lexical_cast and 200$

Subject: Re: [boost] [conversion] try_lexical_cast and 200$
From: Peter Dimov (lists_at_[hidden])
Date: 2013-12-12 10:10:58

Krzysztof Czainski wrote:
> > bool parse( X& x, std::string a, std::string b )
> > {
> >    return try_lexical_cast( a, x.a ) && try_lexical_cast( b, x.b );
> > }
>
> True, but I don't think you should leave it that simple. When parsing of a 
> succeeds, and parsing of b fails, you get x with new value of a, and old 
> value of b.

Actually you get an unspecified value in x.b, I think. Which is mirrored in 
what parse does - you get an unspecified value in x when it fails. If you 
want a strong guarantee:

bool parse( X& x, std::string a, std::string b )
{
    X x2;

    if( try_lexical_cast( a, x2.a ) && try_lexical_cast( b, x2.b ) )
    {
        x = x2;
        return true;
    }
    else
    {
        return false;
    }
}

Note that try_lexical_cast providing that same strong guarantee of leaving 
"out" untouched on failure doesn't simplify the implementation of the strong 
parse.