id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
12016	x3::symbols moves result and therefore removes it from the symbol_parser itself	UsYer	Joel de Guzman	"So, I ran into strange behaviour with boost::spirit::x3 as supplied in boost 1.59:

{{{
#!div
I defined a 'dynamic' symbol table via:
  {{{#!c++
   struct instructions : x3::symbols<OpCode> {
		instructions()
		{
			name(""instructions"");
		}

		void set_instruction_set(const std::unordered_map<std::string, OpCode>& instruction_set) {
			for (const auto& var : instruction_set) {
				add(var.first, var.second);
			}
		}
	} instructions_parser;
  }}}
}}}

{{{
#!div
''OpCode'' is defined as :
  {{{#!c++
    struct OpCode
    {
	    std::string mnemonic;
	    std::vector<...> variants;// actual type in vector<> not important.
    };

  }}}
}}}

now, with the symbol table embedded in the neccessary rules, when parsing an input string like

{{{
#!div

  {{{#!asm
    mov	r2	r1
    mov	r1	@80
  }}}
}}}

the resulting ast only contains the first `mov` with its operands.
The second mov is missing but the operands are correctly parsed.
This may look as followed, when printing the resulting AST:

{{{
#!div

  {{{#!asm
    mov r2 r1 
        r1 @80
  }}}
}}}

{{{
#!div
With the debugger I located the source of the error in symbols.hpp in ''symbol_parser::parse()'':
  {{{#!c++:
    template <typename Iterator, typename Context, typename Attribute>
        bool parse(Iterator& first, Iterator const& last
          , Context const& context, unused_type, Attribute& attr) const
        {
            x3::skip_over(first, last, context);

            if (value_type* val_ptr
                = lookup->find(first, last, get_case_compare<Encoding>(context)))
            {
                x3::traits::move_to(*val_ptr, attr); //<- the error originates from here
                return true;
            }
            return false;
        }

  }}}
}}}

{{{
#!div
with ''move_to'' beeing:
  {{{#!c++:
    template <typename T>
    inline void move_to(T& src, T& dest)
    {
        if (boost::addressof(src) != boost::addressof(dest))
            dest = std::move(src);
    }
  }}}
}}}

As you can see, the ''src'' which is my OpCode instance added in the symbol_parser is moved. This means after the first call its empty again and that's why only the first instructions appears. It's, simply put, moved out of the symbol table.

[http://melpon.org/wandbox/permlink/XxPCnmWExNkmsmP2 '''Live example (shows the bug is also present in 1.60)''']
provided by cv_and_he @stackoverflow


'''Temporary Workaround:'''

By declaring the template parameter as const one can suppress move semantics. The copy-ctor is then called.

{{{
#!div
I.E.:
  {{{#!c++:
 x3::symbols<const OpCode>
}}}
}}}"	Bugs	closed	To Be Determined	spirit	Boost 1.60.0	Regression	fixed	spirit, x3, symbols