[SERIALIZATION] added support for std::variant (#2362)

* [SERIALIZATION] addes support for std::variant

* [SERIALIZATION] bug fix + added tests

* support immutable types

* put an immutable type in std::variant
Co-authored-by: pf <pf@me>

dlib/serialize.h

@@ -89,6 +89,7 @@
         - std::complex
         - std::unique_ptr
         - std::shared_ptr
+        - std::variant (if C++17 is used)
         - dlib::uint64
         - dlib::int64
         - float_details
@@ -119,6 +120,7 @@
         - std::complex
         - std::unique_ptr
         - std::shared_ptr
+        - std::variant (if C++17 is used)
         - dlib::uint64
         - dlib::int64
         - float_details
@@ -233,6 +235,9 @@
 #include <limits>
 #include <type_traits>
 #include <utility>
+#if __cplusplus >= 201703L
+#include <variant>
+#endif
 #include "uintn.h"
 #include "interfaces/enumerable.h"
 #include "interfaces/map_pair.h"
@@ -1216,6 +1221,59 @@ namespace dlib
         { throw serialization_error(e.info + "\n   while deserializing object of type std::tuple"); }
     }
     
+// ----------------------------------------------------------------------------------------
+    
+#if __cplusplus >= 201703L
+    
+    namespace detail
+    {
+        template<typename Variant, std::size_t I = 0>
+        void deserialize_variant_helper(Variant& item, std::istream& in, std::size_t index)
+        {
+            if constexpr (I < std::variant_size_v<Variant>)
+            {
+                if (I == index)
+                {
+                    auto& x = item.template emplace<std::variant_alternative_t<I,Variant>>();
+                    deserialize(bin, x);
+                }
+                else
+                {
+                    deserialize_variant_helper<Variant, I+1>(item, in, index);
+                }
+            }
+            else
+            {
+                throw serialization_error("deserialize_variant_helper() index is out of range of variant size");
+            }
+        }
+    }
+    
+    template<typename... Types>
+    void serialize(
+        const std::variant<Types...>& item,
+        std::ostream& out
+    )
+    {
+        serialize(item.index(),  out);
+        std::visit([&out](const auto& x) {
+            serialize(x, out);
+        }, item);
+    }
+    
+    template <typename... Types>
+    void deserialize(
+        std::variant<Types...>& item,
+        std::istream& in
+    )
+    {
+        std::size_t index = 0;
+        deserialize(index, in);
+        detail::deserialize_variant_helper(item, in, index);
+    }
+    
+#endif
+    
 // ----------------------------------------------------------------------------------------
     
     template <typename domain, typename range, typename compare, typename alloc>

dlib/test/serialize.cpp

@@ -414,6 +414,19 @@ namespace
         return l && r ? *l == *r : l == r;
     }
     
+    struct immutable_type
+    {
+        immutable_type() = default;
+        immutable_type(const immutable_type& other)             = delete;
+        immutable_type& operator=(const immutable_type& other)  = delete;
+        immutable_type(immutable_type&& other)                  = delete;
+        immutable_type& operator=(immutable_type&& other)       = delete;
+        
+        friend void serialize(const immutable_type& x, std::ostream& out) {}
+        friend void deserialize(immutable_type& x, std::istream& in) {}
+        bool operator==(const immutable_type& other) const {return true;}
+    };
+    
     struct my_custom_type
     {
         int a;
@@ -434,15 +447,28 @@ namespace
         std::shared_ptr<string> ptr_shared1;
         std::shared_ptr<string> ptr_shared2;
         std::vector<std::complex<double>> p; 
+#if __cplusplus >= 201703L
+        std::variant<int,float,std::string,immutable_type> q;
+#endif
 
         bool operator==(const my_custom_type& rhs) const
         {         
+#if __cplusplus >= 201703L
+            const bool cpp17_ok = q == rhs.q;
+#else
+            const bool cpp17_ok = true;
+#endif
             return std::tie(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p) == std::tie(rhs.a,rhs.b,rhs.c,rhs.d,rhs.e,rhs.f,rhs.g,rhs.h,rhs.i,rhs.j,rhs.k,rhs.l,rhs.m,rhs.n,rhs.o,rhs.p)
+                    && cpp17_ok
                     && pointers_values_equal(ptr_shared1, rhs.ptr_shared1)
                     && pointers_values_equal(ptr_shared2, rhs.ptr_shared2);
         }
 
+#if __cplusplus >= 201703L
+        DLIB_DEFINE_DEFAULT_SERIALIZATION(my_custom_type, a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, ptr_shared1, ptr_shared2, q);
+#else
         DLIB_DEFINE_DEFAULT_SERIALIZATION(my_custom_type, a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, ptr_shared1, ptr_shared2);
+#endif
     };
 
     struct my_custom_type_array
@@ -1124,7 +1150,9 @@ namespace
         dlib::rand rng(std::time(NULL));
         for (int i = 0 ; i < 1024 ; i++)
             t1.p.push_back(rng.get_random_gaussian());
-
+#if __cplusplus >= 201703L
+        t1.q = "hello there from std::variant, welcome!";
+#endif
         t2.a = 2;
         t2.b = 4.0;
         t2.c.resize(10);