Re: [Boost-users] Metaprogramming Question

Subject: Re: [Boost-users] Metaprogramming Question
From: Nat Linden (nat_at_[hidden])
Date: 2012-02-12 21:37:00

On Sat, Feb 11, 2012 at 2:23 AM, John M. Dlugosz <mpbecey7gu_at_[hidden]>wrote:

On 2/10/2012 7:35 AM, Nat Linden wrote:
>
>> On Thu, Feb 9, 2012 at 11:05 PM, John M. Dlugosz<mpbecey7gu_at_[hidden]**>
>>  wrote:
>>
>>  On 2/9/2012 1:49 PM, Nat Linden wrote:
>>>
>>
>>  template<typename FUNCTOR>
>>>> void ourmacro_f(const FUNCTOR&  f)
>>>> {
>>>>     f(hidden_ostream_object);
>>>> }
>>>>
>>>> #define OURMACRO(EXPRESSION) (ourmacro_f(boost::lambda::_1<**<
>>>> EXPRESSION))
>>>>
>>>
>>  How is that different from just using:
>>>
>>>        #define OURMACRO(EXPRESSION) hidden_stream_object<<  EXPRESSION
>>>
>>
>> Simply that hidden_stream_object really isn't visible in the calling
>> context. It's local to the context of ourmacro_f().
>>
>
> Sorry, I'm still not seeing the point.  The name hidden_stream_object is
> not in scope here but ourmacro_f is instead?  So what?
>
> Or, a plain function to return the stream object?
>
>        ostream& get_stream() { return hidden_stream_object; }
>
>        #define OURMACRO(EXPRESSION) get_stream()<< EXPRESSION

Okay, so I've oversimplified the example code. The point is that the
boost::lambda::_1 placeholder defers the evaluation, giving you extra
control over the operation. Put differently, the expression is evaluated in
the context of your function rather than in the calling context. Consider:

template<typename FUNCTOR>
void ourmacro_f(const FUNCTOR&  f)
{
    if (some_condition)
    {
        f(hidden_ostream_object);
    }
}

or how about:

template<typename FUNCTOR>
void ourmacro_f(const char* expr_string, const FUNCTOR&  f)
{
    try
    {
        f(hidden_ostream_object);
    }
    catch (const std::exception& e)
    {
        hidden_ostream_object << "Exception " << typeid(e).name() << "
evaluating (" << expr_string << "): " << e.what() << std::endl;
    }
}

#define OURMACRO(EXPRESSION) (ourmacro_f(#EXPRESSION, boost::lambda::_1 <<
EXPRESSION))