Re: Question on STL Function adaptors

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++,comp.sources.d

Date:

Sun, 15 Mar 2009 10:05:34 +0100

Message-ID:

<gpigcv$qcq$1@news.motzarella.org>

* masood.iqbal@lycos.com:

Hi,

I am a beginner at STL and I am having a lot of trouble understanding
function adaptors. My team lead has given me the following cheat
sheet:

(1) If f = bind1st(g,x)
then f(y) means g(x,y)

(2) If f = bind2nd(g,x)
then f(y) means g(y,x)

(3) If f = mem_fun(mf)
then f(p) means p->mf()

(4) If f = mem_fun_ref(mf)
then f(r) means r.mf()

(5) If f = not1(g)
then f(x) means !g(x)

(6) If f = not2(g)
then f(x,y) means !g(x,y)

The problem is that in most of the examples of say bind2nd that I am
seeing, there is no "y" being passed [as in f(y) means g(y,x) in the
TL's cheat sheet].

Here's an example usage:
find_if(v.begin(), v.end(), bind2nd(greater<int>(), 5))
In this case:
- "greater<int>()" is the same as "g" in the cheat sheet
- 5 is x in the cheat sheet

But where is y (from the cheat sheet) being passed?

find_if calls the adaptor, where in each call the argument will be an element
from the collection defined by the iterators you pass in (namely here, v.begin()
and v.end()).

By the way, note that the standard library's adaptors are horribly restricted,
they have severe problems with references.

You should instead use e.g. Boost's adaptors.

Cheers & hth.,

- Alf

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