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Re: C++ token move constructors/assignment with LALR

From: Anthony Heading
Subject: Re: C++ token move constructors/assignment with LALR
Date: Thu, 21 Apr 2022 17:15:33 +0900
User-agent: Cyrus-JMAP/3.7.0-alpha0-568-g521196dd5d-fm-20220414.001-g521196dd

On Thu, Apr 21, 2022, at 1:57 AM, Frank Heckenbach wrote:
> The comment you quoted seems to refer only to the move assignment
> operators, not the move constructors which are conditional on
> "201103L <= YY_CPLUSPLUS" instead.

Totally right, yes it is only the move assignment which was missing / now 
hidden,  and that's actually the most immediate problem.   Assuming of course 
it's agreed to be a problem at all,  as I concur the skeleton doesn't use it.   
 I think the issue arises if the user wants to assign the tokens (which I think 
is natural, e.g. for lexer putback and so on) and starts even with simple 

%language "c++"
%skeleton "lalr1.cc"
%define api.token.constructor
%define api.value.type variant

%token <int> INT


#include <base/base.h>
#include "foo.y.h"

using namespace yy;

parser::symbol_type yylex();



start :


void parser::error(const string &) { }

parser::symbol_type yylex()
    return parser::make_INT(12345);

int main() {
    parser::symbol_type sym;
    sym = yylex();
    return 0;

foo.y:34:9: error: object of type 'parser::symbol_type' cannot be assigned 
because its copy assignment operator is implicitly deleted
    sym = yylex();
obj/foo.y.h:625:26: note: copy assignment operator of 'symbol_type' is 
implicitly deleted because base class 'basic_symbol<yy::parser::by_kind>' has a 
deleted copy assignment operator
    struct symbol_type : basic_symbol<by_kind>
obj/foo.y.h:475:7: note: copy assignment operator is implicitly deleted because 
'basic_symbol<yy::parser::by_kind>' has a user-declared move constructor
      basic_symbol (basic_symbol&& that)
1 error generated.


So because there is no move assignment operator,  C++ tried to fall back to 
(implicit) copy assignment, but it can't even manage that because there IS a 
move constructor.

Enabling the move assignment operator in the skeleton fixes this,  and works 
for unique_ptr etc too.

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