Re: Template argument deduction

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Sun, 29 Jan 2012 14:07:48 -0800 (PST)
Am 28.01.2012 00:17, schrieb Ulrich Eckhardt:

Am 27.01.2012 08:48, schrieb Daniel Kr?gler:

one special example which allow to deduce a return type


it does not solve the OPs problem, though.

I beg to differ, you just have to return a proxy type with a template
conversion operator:

#include <iostream>
#include <ostream>

struct X
// actual template function to be called
template<typename T> T call() const
{ std::cout << "call<" << typeid(T).name() << ">()\n"; }

// proxy type
struct P
explicit P(X const& x): m_x(x) {}
template<typename T> operator T() const
{ return<T>(); }
X const& m_x;

/* Attention: This doesn't call any function, it only
creates a proxy. */
P operator()() const
{ return P(*this); }

int main()
X x;

int vi = x();
float vf = x();

A big danger here is that no function is called if the result of x() is
not used anywhere. However, I don't think this is used in places where
you rely on the side effects of calling a function.

Nice idea, Uli. It is a bit tricky to get this work for the OP's
example, the main reason being a potential ambiguity when the conversion
function is provided to the templated converting pair constructor. With
some usage of constraints on the conversion function it should be doable
in the following way:

#include <map>
#include <memory>
#include <type_traits>

template<class T>
struct is_shared_ptr
   static const bool value = false;

template<class T>
struct is_shared_ptr<std::shared_ptr<T> >
   static const bool value = true;

struct HeapConvertInserter
    template <class Res, class Key>
    typename std::enable_if<is_shared_ptr<Res>::value, Res>::type
    call(const Key& key) const
      return Res(new typename Res::element_type(key));

    template<class Key>
    struct Proxy
       template<class T, class = typename
       operator T() const

       friend class HeapConvertInserter;
       const HeapConvertInserter& ins;
       const Key& key;
       Proxy(const HeapConvertInserter& ins, const Key& key): ins(ins),
key(key) {}

    template<class Key>
    Proxy<Key> operator()(const Key& key) const
    { return Proxy<Key>(*this, key); }

int main()
   HeapConvertInserter delegate;
   std::map<int, std::shared_ptr<float> > table;
   int key = 12;
   table.insert(std::map<int, std::shared_ptr<float> >::value_type(key,

Greetings from Bremen,

Daniel Kr?gler

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