Re: compound attribute rules - pair FAQ

On Tue, 14 Jun 2016 12:18:16 +0100
Patrick Welche <[email protected]> wrote:

> After the ease of writing a parser with spirit, I'm getting stuck on
> working out a compatible type for the generated AST. (The idea is to
> avoid semantic actions, right?)
> 
> To figure out the basic building blocks, I just tried
> 
>     +alnum >> ' ' >> +alnum
> 
> which I think gives
> 
>     vector<char>     Unused    vector<char>
> 
> aka
>     string Unused string
> 
> But then, can one carry on and remove Unused
>     a: A, b: Unused --> (a >> b): A
> 
>     string string
>     tuple <string, string>
>     pair <string, string>
> ?
> 
> The attached simple experiment suggests not, as "fails.cpp" doesn't
> compile, works.cpp does.
> 
> What am I missing? Some other error?
> 
> 

Adding the relevant code that was attached:

> struct result
> {
> 	std::pair<std::string, std::string> both;
> };
>
> BOOST_FUSION_ADAPT_STRUCT(result,
> 	both
> )
>
> int main()
> {
> 	std::string buf("one two");
> 	std::string::const_iterator iter = buf.begin();
> 	std::string::const_iterator end = buf.end();
> 	result res;
> 	bool ok = x3::parse(iter, end, +x3::alnum >> ' ' >>
> +x3::alnum, res); if (!ok)
>		std::cout << "Parsing failed\n";
>	else if (iter != end)
>		std::cout << "Bit left over at the end: "
>		          << std::string(iter, end) << std::endl;
>	else
>		std::cout << "found: " << res.both.first << ' '
>		                       << res.both.second << std::endl;
>
>	return 0;
> }

The problem is the "double" wrapping. An adapted struct _is_ a
sequence, and so is an adapted `std::pair`. So the attribute provided
to the parse function is `sequence<sequence<std::string, std::string>>`
which is incorrect. Use the pair from the other code snippet or put
both `std::string`s in the struct directly:

    struct result
    {
        std::string one;
        std::string two;
    };

    BOOST_FUSION_ADAPT_STRUCT(result, one, two);

Lee

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