Template function to return tuple values (different types) via ENUM lookup

Question

the following code produces

"error: invalid operands of types '' and 'const size_t {aka const long unsigned int}' to binary 'operator<'" (gcc-4.9.1)

I just want a lookup function for default values of different types. Is this supposed to work at all?

#include <iostream>
#include <string>
#include <tuple>

// IDs
enum class ID : size_t
{
    AAA, // 0
    BBB, // 1
    CCC, // 2
    DDD  // 3
};

// default values for each ID
const auto defaultValsForIDs = std::make_tuple(
        int(1),             // 0
        std::string("bbb"), // 1 
        double(3.5),        // 2
        int(-5)             // 3
);


//------------------------------------------------------
// HERE IS WHERE IT GETS MESSY:
//------------------------------------------------------
// default values for each deviceID
template<typename EnumT>
using underlayingEnumT = typename std::underlying_type<EnumT>::type;

template<typename EnumT>
constexpr underlayingEnumT<EnumT> to_underlying(EnumT e) 
{
    return static_cast<underlayingEnumT<EnumT>>(e);
}

template<typename EnumT>
auto getDefaultValue(const EnumT e) 
-> decltype(std::get<to_underlying<EnumT>(e)>(defaultValsForIDs)) // <- THIS WON'T WORK
{
    return std::get<to_underlying<EnumT>(e)>(defaultValsForIDs);
}


//------------------------------------------------------
// THIS DOES NOT COMPILE AS WELL
//------------------------------------------------------
template<>
auto getDefaultValue(const size_t xx) 
-> decltype(std::get<xx>(defaultValsForIDs)) // <- THIS WON'T WORK
{
    return std::get<xx>(defaultValsForIDs);
}

int main(int , char** )
{
    std::cout << getDefaultValue(ID::AAA) << std::endl;

    return 0;
}

Am I missing a template somewhere? see Where and why do I have to put the "template" and "typename" keywords? or Compile error: unresolved overloaded function type

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EDIT as suggested by Piotr:

changed:

template<typename EnumT, EnumT e>
auto getDefaultValue() 
-> decltype(std::get<to_underlying<EnumT>(e)>(defaultValsForIDs))
{
    return std::get<to_underlying<EnumT>(e)>(defaultValsForIDs);
}

and added instance for ID:

template<ID id>
auto getDefaultValForID() 
-> decltype(getDefaultValue<ID, id>())
{
    return getDefaultValue<ID,id>();
}

then:

int main()
{
    std::cout << getDefaultValForID<ID::BBB>() << std::endl;

    return 0;
}

works like a charm.

I would guess that a function must always return the same type. For your second example, the return type depends on xx. For your first example, it depends on e (not on EnumT). — Petr

Piotr Skotnicki Piotr Skotnicki · Accepted Answer · 2015-10-01T10:28:52

A function parameter is not a constant expression, thus it cannot be used as a non-type template argument. Instead, you need to lift it up to a template parameter list:

template <std::size_t xx>
auto getDefaultValue() 
    -> decltype(std::get<xx>(defaultValsForIDs))
{
    return std::get<xx>(defaultValsForIDs);
}

template <typename EnumT, EnumT e>
auto getDefaultValue() 
    -> decltype(std::get<to_underlying<EnumT>(e)>(defaultValsForIDs))
{
    return std::get<to_underlying<EnumT>(e)>(defaultValsForIDs);
}

and then use it like:

getDefaultValue<ID, ID::AAA>()

or

getDefaultValue<0>()

DEMO

Template function to return tuple values (different types) via ENUM lookup

1 Answers