Re: [boost] [scope_exit] MSVC error C2355: 'this' : can only be referenced inside non-static member functions

Subject: Re: [boost] [scope_exit] MSVC error C2355: 'this' : can only be referenced inside non-static member functions
From: lcaminiti (lorcaminiti_at_[hidden])
Date: 2011-03-01 11:38:38

>>On Thu, August 26, 2010 12:49 am, Lorenzo Caminiti wrote:
>>
>> I am trying to use Boost.ScopeExit parameter binding mechanism to
>> implement Boost.LocalFunction. To do so, I need to use some internal
>> ScopeExit code to detect the type of `this` and bind its value to the
>> local function declaration.
>>
>> <snip>
>>
>> 2) Do you know of a workaround for this problem?
>>
>I posted a workaround for this a while back
>(http://lists.boost.org/Archives/boost/2009/03/149540.php) which used
>type indexing (where the index for typeof(this) is captured outside of a
>template parameter list) to avoid specifying 'this' in a template
>parameter list.  The mechanics for the solution came from boost.typeof. 
>
> <snip>
>
>I have attached my latest version of the patch which still applies fine
>to 1.44 and, I think, works on cl versions from vc7.1 upwards.
>
>You should be able to use the same type index trick in localfunction. 
>The key definition is the function-style macro
>BOOST_SCOPE_EXIT_TYPEDEF_TYPEOF_THIS().

Why is this patch defining the following accessors for the ScopeExit local
struct?

+#define BOOST_SCOPE_EXIT_DEFINE_THIS_ACCESSOR_this(id) \
+        inline BOOST_PP_CAT(se_this_type,id) operator->() { return
boost_se_params_->se_this; } \
+        inline BOOST_PP_CAT(se_this_type,id) operator*()  { return
boost_se_params_->se_this; } \
+        inline operator BOOST_PP_CAT(se_this_type,id)()   { return
boost_se_params_->se_this; } \

I think these are meant to use `this` to access the bound object instead of
the local struct object. However, these accessors don't work because
`this->` (and `*this`) never calls the overloaded `operator->`:

#include <iostream>

struct obj_t {
    int x;
    int f() { return -1; }
} obj;

typedef obj_t* this_t;

int main() {
    struct l {
        void operator()() {
            std::cout << this->x << std::endl; // error :((
            std::cout << this->f() << std::endl; // error :((
            
            std::cout << this->operator->()->x << std::endl; // ok
            std::cout << this->operator->()->f() << std::endl; // ok
        }
        this_t operator->() { return &obj; }
        this_t operator*() { return &obj; }
        operator this_t() { return &obj; }
        int f;
    } ll;
    ll();
    return 0;
}

Am I not understanding the purpose of the patch's assessors correctly?

Thanks for the clarification.

--
Lorenzo
-- 
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