C++ SFINAE partial specialization

user_1870801 0
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I've been trying to define an auxiliary class to help me work with template methods in which I would like a generic implementation both for complex and real types.

That has been my attempt so far:

#include<type_traits>
#include<complex>


template<class T>
struct is_complex{ static constexpr bool value = false;};

template<class T>
struct is_complex<std::complex<T>> : 
    std::integral_constant<bool,
    std::is_integral<T>::value ||
    std::is_floating_point<T>::value>{};

template<class T>
struct is_arithmetic:
    std::integral_constant<bool,
    std::is_integral<T>::value ||
    std::is_floating_point<T>::value ||
    is_complex<T>::value>{};


template<class T,
typename std::enable_if_t<is_arithmetic<T>::value,int> =0>
struct real_type {typedef T type;};

template<class T>
struct real_type<typename std::complex<T>>{typedef T type;};

I want to get something like

typename real_type<std::complex<double>> myVar1;//myVar1 is double
typename real_type<double> myVar2;//myVar2 is double

I was able to make it work as long as I didn't care that non-arithmetic types also had real_type<T>::type. But now that I have added this additional constraint, I cannot make it work and I don't really see why.

To clarify: I would like that calls like real_type<std::string>::type would generate compile-time errors. I want these calls to be valid only to arithmetic (including complex) and integral types.

The compiler error of my latest attempt was:

non-type template argument specializes a template parameter with dependent type 'typename std::enable_if_t<is_arithmetic<T>::value, int>' (aka 'typename enable_if<is_arithmetic<T>::value, int>::type')

But I don't know how to deal with it. If this information is useful, I have access to compilers supporting C++17.

template-specialization c++17 sfinae c++
2 Answers

Usually this is done with specialization and a template defaulted parameter.

I mean

template <typename, typename = void>
struct real_type;

template <typename T>
struct real_type<T, std::enable_if_t<std::is_arithmetic_v<T>>>
 { using type = T; };

template <typename T>
struct real_type<std::complex<T>, void>
 { using type = T; };

where you have a separate specialization for std::complex and, as observed by Patrick Roberts (thanks), without std::complex your is_arithmetic become a duplicate of std::is_arithmetic (so is better directly use std::is_arithmetic).

You get

real_type<int>                 r1;   // compile
real_type<std::complex<float>> r2;   // compile
//real_type<std::string>         r3; // compilation error
user_60226560

May I suggest a slightly more concise implementation that works more generally (which requires c++17):

#include <type_traits>

// catch all for single parameters that have 0 template parameters 
template <typename T>
struct real_type{
    static_assert(std::is_arithmetic_v<T>);
    using type = T;
};
template <typename T>
using real_type_t = typename real_type<T>::type;

// magically catch anything which matches V<T, Ts...> and exposes T as `type`
template <template <typename...> typename V, typename T, typename...Ts>
struct real_type<V<T, Ts...>>{
    using type = real_type_t<T>;
};

#include <vector>
#include <complex>

using d = real_type_t<double>;
static_assert(std::is_same_v<d, double>);
using d2 = real_type_t<std::vector<double>>;
static_assert(std::is_same_v<d2, double>);
using d3 = real_type_t<std::complex<double>>;
static_assert(std::is_same_v<d3, double>);

// doesn't compile
struct NotValid {};
using d4 = real_type_t<std::vector<NotValid>>;
user_79112350
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