Type inference in Visual C ++ 2008

Question

In Microsoft Visual C ++ 2008, is there some kind of mechanism for choosing the type of a specific provider, similar to standardized autoor decltypein C ++ 0x?

Answer 1

No, nothing similar, standard and not specific to the supplier, nor the add-on. You will need to upgrade to VS2010, it implements auto.

Answer 2

Quote from Boost listing list article by Arkady Vertlib:

God only knows what else can be found inside this compiler if you want to dig real depth.

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http://rsdn.ru/Forum/?mid=1094305

, :

// type_of() evil implementation for VC7
//
// (c) Chez
// mailto:[e-mail]

#include "stdafx.h"

// This file contains:
// 1) type_id(type)
// 2) var_type_id(expersssion)
// 3) type_of(expression)

// IMPLEMENTATION
template<int ID>
class CTypeRegRoot
{
public:
    class id2type;
};

template<typename T, int ID>
class CTypeReg : public CTypeRegRoot<ID>
{
public:
    class CTypeRegRoot<ID>::id2type // This uses nice VC6-VC7 bugfeature
    {
    public:
        typedef T Type;
    };

    typedef void Dummy;
};

template<int N>
class CCounter;

// TUnused is required to force compiler to recompile CCountOf class
template<typename TUnused, int NTested = 0>
class CCountOf
{
public:
    enum
    {
        __if_exists(CCounter<NTested>) { count = CCountOf<TUnused, NTested + 1>::count }
        __if_not_exists(CCounter<NTested>) { count = NTested }
    };
};

template<class TTypeReg, class TUnused, int NValue> // Helper class
class CProvideCounterValue
{
public:
    enum { value = NValue };
};

// type_id
#define type_id(type) \
    (CProvideCounterValue< \
        /*register TYPE--ID*/ typename CTypeReg<type, CCountOf<type >::count>::Dummy, \
        /*increment compile-time Counter*/ CCounter<CCountOf<type >::count>, \
        /*pass value of Counter*/CCountOf<type >::count \
     >::value)

// Lets type_id() be > than 0
class __Increment_type_id { enum { value = type_id(__Increment_type_id) }; };

template<int NSize>
class sized
{
private:
    char m_pad[NSize];
};

template<typename T>
typename sized<type_id(T)> VarTypeID(T&);
template<typename T>
typename sized<type_id(const T)> VarTypeID(const T&);
template<typename T>
typename sized<type_id(volatile  T)> VarTypeID(volatile T&);
template<typename T>
typename sized<type_id(const volatile T)> VarTypeID(const volatile T&);

// Unfortunatelly, var_type_id() does not recognize references
#define var_type_id(var) \
    (sizeof(VarTypeID(var)))

// type_of
#define type_of(expression) \
    /* This uses nice VC6-VC7 bugfeature */ \
    CTypeRegRoot<var_type_id(expression)>::id2type::Type

// auto_operator
#define auto_operator(arg1, arg2, op) \
    type_of(instance(arg1) op instance(arg2)) operator op


// TEST    
class A
{
public:
    friend static const char* operator +(const A& a, const A& b)
    {
        return "chijik-pijik";
    }
};

template<typename T>
class Plus
{
public:
    friend static type_of(T() + T()) operator +(const Plus<T>& a, const Plus<T>& b)
    {
        return a.m + b.m;
    }

    T m;
};

int _tmain(int argc, _TCHAR* argv[])
{
    Plus<A> a1, a2;
    const char* x = a1 + a2;

    return 0;
}

, , , , MSVC 7.x. , . , ?

hth.,

Answer 3

BOOST. , BOOST, , Visual Studio 2008 (, , ):

namespace typeid_detail {

template <int N>
struct encode_counter : encode_counter<N - 1> {};

template <>
struct encode_counter<0> {};

char (*encode_index(...))[5];
// need to default to a larger value than 4, as due to MSVC ETI errors. (sizeof(int) = 4)

struct msvc_extract_type_default_param {};

template <typename ID, typename T = msvc_extract_type_default_param>
struct msvc_extract_type;

template <typename ID>
struct msvc_extract_type<ID, msvc_extract_type_default_param> {
    template <bool>
    struct id2type_impl;

    typedef id2type_impl<true> id2type;
};

template <typename ID, typename T>
struct msvc_extract_type : msvc_extract_type<ID,msvc_extract_type_default_param> {
    template <>
    struct id2type_impl<true> { // VC8.0 specific bugfeature
        typedef T type;
    };

    template <bool>
    struct id2type_impl;

    typedef id2type_impl<true> id2type;
};

template <typename T, typename ID>
struct msvc_register_type : msvc_extract_type<ID, T> {
};

template <int i>
struct int_ {
    enum { value = i };
};

template <int ID>
struct msvc_typeid_wrapper {
    typedef typename msvc_extract_type<int_<ID> >::id2type id2type;
    typedef typename id2type::type type;
};

template <>
struct msvc_typeid_wrapper<1> {
    typedef msvc_typeid_wrapper<1> type;
};
// workaround for ETI-bug for VC6 and VC7

template <>
struct msvc_typeid_wrapper<4> {
    typedef msvc_typeid_wrapper<4> type;
};
// workaround for ETI-bug for VC7.1

#define TYPEOF_INDEX(T) (sizeof(*encode_index((encode_counter<405/*1005*/>*)0))) // this needs to be lower for VS 2008, otherwise causes too deep templates
#define TYPEOF_NEXT_INDEX(next) friend char (*encode_index(encode_counter<next>*))[next];

template <typename T>
struct encode_type {
    static const unsigned value = TYPEOF_INDEX(T);
    // get the next available compile time constants index

    typedef typename msvc_register_type<T,int_<value> >::id2type type;
    // instantiate the template

    static const unsigned next = value + 1;
    // set the next compile time constants index

    TYPEOF_NEXT_INDEX(next);
    // increment the compile time constant (only needed when extensions are not active)
};

template <class T>
struct sizer {
    typedef char(*type)[encode_type<T>::value];
};

template <typename T> typename sizer<T>::type encode_start(T const&);
// a function that converts a value to size-encoded type (not implemented, only needed for type inference)

template <typename Organizer, typename T>
msvc_register_type<T, Organizer> typeof_register_type(const T&, Organizer* = 0);

} // ~typeid_detail

#define type_of(expr) \
    typeid_detail::msvc_typeid_wrapper<sizeof(*typeid_detail::encode_start(expr))>::type

, , :

int x = 123;
float y = 456.0f;
typedef type_of(x+y) int_plus_float;
int_plus_float value = x + y;

, , BOOST .