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Do not unconditionally redefine C++14 constructs #2533

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Dec 16, 2020
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Do not unconditionally redefine C++14 constructs #2533

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24 changes: 20 additions & 4 deletions include/nlohmann/detail/meta/cpp_future.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2,19 +2,32 @@

#include <cstddef> // size_t
#include <type_traits> // conditional, enable_if, false_type, integral_constant, is_constructible, is_integral, is_same, remove_cv, remove_reference, true_type
#include <utility> // index_sequence, make_index_sequence, index_sequence_for

#include <nlohmann/detail/macro_scope.hpp>

namespace nlohmann
{
namespace detail
{
// alias templates to reduce boilerplate
template<bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;

template<typename T>
using uncvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;

// implementation of C++14 index_sequence and affiliates
#ifdef JSON_HAS_CPP_14

// the following utilities are natively available in C++14
using std::enable_if_t;
using std::index_sequence;
using std::make_index_sequence;
using std::index_sequence_for;

#else

// alias templates to reduce boilerplate
template<bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;

// source: https://stackoverflow.com/a/32223343
template<std::size_t... Ints>
struct index_sequence
Expand Down Expand Up @@ -45,6 +58,8 @@ template<> struct make_index_sequence<1> : index_sequence<0> {};
template<typename... Ts>
using index_sequence_for = make_index_sequence<sizeof...(Ts)>;

#endif

// dispatch utility (taken from ranges-v3)
template<unsigned N> struct priority_tag : priority_tag < N - 1 > {};
template<> struct priority_tag<0> {};
Expand All @@ -58,5 +73,6 @@ struct static_const

template<typename T>
constexpr T static_const<T>::value;

} // namespace detail
} // namespace nlohmann
25 changes: 21 additions & 4 deletions single_include/nlohmann/json.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2736,19 +2736,33 @@ class other_error : public exception

#include <cstddef> // size_t
#include <type_traits> // conditional, enable_if, false_type, integral_constant, is_constructible, is_integral, is_same, remove_cv, remove_reference, true_type
#include <utility> // index_sequence, make_index_sequence, index_sequence_for

// #include <nlohmann/detail/macro_scope.hpp>


namespace nlohmann
{
namespace detail
{
// alias templates to reduce boilerplate
template<bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;

template<typename T>
using uncvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;

// implementation of C++14 index_sequence and affiliates
#ifdef JSON_HAS_CPP_14

// the following utilities are natively available in C++14
using std::enable_if_t;
using std::index_sequence;
using std::make_index_sequence;
using std::index_sequence_for;

#else

// alias templates to reduce boilerplate
template<bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;

// source: https://stackoverflow.com/a/32223343
template<std::size_t... Ints>
struct index_sequence
Expand Down Expand Up @@ -2779,6 +2793,8 @@ template<> struct make_index_sequence<1> : index_sequence<0> {};
template<typename... Ts>
using index_sequence_for = make_index_sequence<sizeof...(Ts)>;

#endif

// dispatch utility (taken from ranges-v3)
template<unsigned N> struct priority_tag : priority_tag < N - 1 > {};
template<> struct priority_tag<0> {};
Expand All @@ -2792,6 +2808,7 @@ struct static_const

template<typename T>
constexpr T static_const<T>::value;

} // namespace detail
} // namespace nlohmann

Expand Down