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

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Sat, 19 May 2012 13:42:09 -0700 (PDT)
{ This thread has turned into a debugging effort of a specific piece
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   Please keep that in mind when replying -mod }

Am 19.05.2012 10:28, schrieb Mark Summerfield:

I tried what you suggested but got the same errors.

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<typename C::value_type(*)(const std::string)> validate)
      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 + "'");

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

At the call site I tried:

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

None of which would compile.

The first and third form cannot possibly be well-formed for reasons
that I explained in my very first reply in this thread. The other
forms should work. Since you do not provide a complete program, but
only snippets there might be several reasons for the failure (include
that the compiler version you used cannot properly handle some of the
new C++11 constructs). Except from the lambda expression, it is easy
to make your code a fully C++03 compatible program like so:

#include <string>
#include <set>

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

struct ValueError { ValueError(const std::string& what) {} };

template<typename T>
struct function;

template<typename R, typename T>
struct function<R(T)>
    template<typename F>

template<typename C>
function<typename C::value_type(const std::string)>
makeSetValidator(const C &validItems,
       typename identity<typename C::value_type(*)(const
std::string)>::type validate)
    return function<typename C::value_type(const std::string)>(validate);

template<typename T>
T validate(const std::string s) { return T(); }

int main()
    makeSetValidator(std::set<int>(), validate<int>);
    makeSetValidator(std::set<int>(), validate);

This compiles on every compiler I tested. I recommend to start with
this and to increase the functionality step-wise to the point where it
does no longer work, then try to isolate the actual problem.

HTH & Greetings from Bremen,

Daniel Kr?gler

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