Template function dependent on non-type parameter

2
votes

Is it somehow possible to define an overloaded template function based on a non-type parameter?

following situation:

template<uint8_t> void SetupMem();

template<> void SetupMem<4>()
{ /* some code */ }


template<> void SetupMem<8>()
{ /* some code */ }

void TemplateCaller()
{
   // use the plattform-specific template function at compile-time
   SetupMem<sizeof(size_t)>();
}

now is it somehow possible to change the return value of SetupMem based on the non-type parameter? e.g.:

template<> uint32_t SetupMem<4>(){}
template<> uint64_t SetupMem<8>(){}

So that TemplateCaller() does not explicitly calls SetupMem with the desired template parameter (so avoiding something like: SetupMem<uint64, sizeof(size_t)>();)? Possible solutions upto C++11 are welcome :)

2
c++c++11templatestemplate-function
Isn't this an XY problem? If you're only worried about size_t, then simply use size_t. Note that many libraries these days come in 32 and 64 bit versions. - Walter

2 Answers

3
votes

Just use simple function overloading and std::integral_constant:

std::uint32_t SetupMem(std::integral_constant<int, 4>); // (0)
std::uint64_t SetupMem(std::integral_constant<int, 8>); // (1)

void TemplateCaller()
{
   auto a = SetupMem(std::integral_constant<int, 4>{}); // calls (0)
   auto b = SetupMem(std::integral_constant<int, 8>{}); // calls (1)
}

You can introduce a template type alias for readability:

template <int X>
using ic = std::integral_constant<int, X>;

std::uint32_t SetupMem(ic<4>);
std::uint64_t SetupMem(ic<8>);

void TemplateCaller()
{
   auto a = SetupMem(ic<4>{});
   auto b = SetupMem(ic<8>{});
}

live example on wandbox.org

If your compiler doesn't support integral_constant, all you need to do is define it yourself:

template <int>
struct ic { };

std::uint32_t SetupMem(ic<4>);
std::uint64_t SetupMem(ic<8>);

void TemplateCaller()
{
   auto a = SetupMem(ic<4>{});
   auto b = SetupMem(ic<8>{});
}

live example on wandbox.org

3
votes

You can write a type trait:

template<::std::size_t x_Size> class
t_IntegralFromSize;

template<> class
t_IntegralFromSize<4> final
{
    public: using type = ::std::uint32_t;
};

template<> class
t_IntegralFromSize<8> final
{
    public: using type = ::std::uint64_t;
};

template<::std::size_t x_Size> typename t_IntegralFromSize<x_Size>::type
SetupMem();

template<> t_IntegralFromSize<4>::type
SetupMem<4>()
{ /* some code */ }


template<> t_IntegralFromSize<8>::type
SetupMem<8>()
{ /* some code */ }

void TemplateCaller()
{
    // use the plattform-specific template function at compile-time
    SetupMem<sizeof(size_t)>(); // works without changes
}

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