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

Mark Summerfield <>
Fri, 18 May 2012 06:03:14 -0700 (PDT)
Hi Daniel,

Is what I'm trying to do very much against the grain of C++11? Or is
it that I'm taking the wrong approach?

Scenario: I have validator functions that take a string and return an
object of a given type, e.g., int validator(string); string
validator(string); bool validator(string) --- or throws if the string
can't be converted to the required type (e.g., for int the string
doesn't represent a number or is too big or too small). These work
fine. But I also want to create a validator that takes an existing
validator plus a collection of specific values and returns a new
validator that does the conversion (or throws) and if it hasn't thrown
then checks to see if the converted item is in the collection.

I changed the code along the lines you suggested:

template<typename T>
struct identity { typedef T type; };
template<typename T>
using NonDeduced = typename identity<T>::type;

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

template<typename C>
Validator<typename C::value_type>
makeSetValidator(const C &validItems,
                  NonDeduced<Validator<typename C::value_type>>
     return [=](const std::string s)->typename C::value_type{
         const auto x = validate(s);
         if (std::find(std::begin(validItems), std::end(validItems), x)
             != std::end(validItems))
             return x;
         throw ValueError("Invalid item '" + s + "'");

I also tried changing the call site:

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

But g++ 4.7.0 gave me this:

main.cpp: In function ?void test9(const string&)?:
main.cpp:318:49: error: cannot resolve overloaded function ?validate?
based on conversion to type
?OptArg::NonDeduced<std::function<int(std::basic_string<char>)> > {aka
main.cpp: In function ?void test10(const string&)?:
main.cpp:356:42: error: no matching function for call to
?makeSetValidator(std::set<std::basic_string<char> >, <unresolved
overloaded function type>)?
main.cpp:356:42: note: candidate is:
In file included from optarg_option.hpp:17:0,
                  from optarg.hpp:33,
                  from main.cpp:14:
optarg_validator.hpp:104:1: note: template<class C>
OptArg::Validator<typename C::value_type>
OptArg::makeSetValidator(const C&,
C::value_type(std::basic_string<char>)> >)
optarg_validator.hpp:104:1: note: template argument deduction/
substitution failed:
main.cpp:356:42: note: cannot convert

(std::initializer_list<std::basic_string<char> >{((const

std::basic_string<char>*)(& const std::basic_string<char> [2]
{std::basic_string<char>(((const char*)"a"), (*(const
std::allocator<char>*)(& std::allocator<char>()))),
std::basic_string<char>(((const char*)"bee"), (*(const
std::allocator<char>*)(& std::allocator<char>())))})), 2u}, (*(const
std::less<std::basic_string<char> >*)(&
std::less<std::basic_string<char> >())), (*(const allocator_type*)(&
std::allocator<std::basic_string<char> >())))? (type
?std::set<std::basic_string<char> >?) to type ?const


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