### Custom Specializations

If you try to specialize `json_type_traits` for a type that is already
specialized in the jsoncons library, for example, a custom container 
type that satisfies the conditions for a sequence container, you 
may see a compile error "more than one partial specialization matches the template argument list".
For these situations `jsoncons` provides the traits class
```cpp
template <typename T>
struct is_json_type_traits_declared : public std::false_type {};
```
which inherits from [std::false_type](http://www.cplusplus.com/reference/type_traits/false_type/).
This traits class may be specialized for a user-defined type with member constant `value` equal `true`
to inform the `jsoncons` library that the type is already specialized.  

### Examples

[Extend json_type_traits to support `boost::gregorian` dates.](#A1)  
[Specialize json_type_traits to support a book class.](#A2)  
[Specialize json_type_traits for a container type that the jsoncons library also supports](#A3)  
[Convert JSON to/from boost matrix](#A4)

#### Extend json_type_traits to support `boost::gregorian` dates.

```cpp
#include <jsoncons/json.hpp>
#include "boost/datetime/gregorian/gregorian.hpp"

namespace jsoncons 
{
    template <typename Json>
    struct json_type_traits<Json,boost::gregorian::date>
    {
        static bool is(const Json& val) noexcept
        {
            if (!val.is_string())
            {
                return false;
            }
            std::string s = val.template as<std::string>();
            try
            {
                boost::gregorian::from_simple_string(s);
                return true;
            }
            catch (...)
            {
                return false;
            }
        }

        static boost::gregorian::date as(const Json& val)
        {
            std::string s = val.template as<std::string>();
            return boost::gregorian::from_simple_string(s);
        }

        static Json to_json(boost::gregorian::date val)
        {
            return Json(to_iso_extended_string(val));
        }
    };
}
```
```cpp
namespace ns
{
    using jsoncons::json;
    using boost::gregorian::date;

    json deal = json::parse(R"(
    {
        "Maturity":"2014-10-14",
        "ObservationDates": ["2014-02-14","2014-02-21"]
    }
    )");

    deal["ObservationDates"].push_back(date(2014,2,28));    

    date maturity = deal["Maturity"].as<date>();
    std::cout << "Maturity: " << maturity << '\n' << '\n';

    std::cout << "Observation dates: " << '\n' << '\n';

    for (auto observation_date: deal["ObservationDates"].array_range())
    {
        std::cout << observation_date << '\n';
    }
    std::cout << '\n';
}
```
Output:
```
Maturity: 2014-Oct-14

Observation dates:

2014-Feb-14
2014-Feb-21
2014-Feb-28
```


#### Specialize json_type_traits to support a book class.

```cpp
#include <iostream>
#include <jsoncons/json.hpp>
#include <vector>
#include <string>

namespace ns {
    struct book
    {
        std::string author;
        std::string title;
        double price;
    };
} // namespace ns

namespace jsoncons {

    template <typename Json>
    struct json_type_traits<Json, ns::book>
    {
        using allocator_type = Json::allocator_type;

        static bool is(const Json& j) noexcept
        {
            return j.is_object() && j.contains("author") && 
                   j.contains("title") && j.contains("price");
        }
        static ns::book as(const Json& j)
        {
            ns::book val;
            val.author = j.at("author").template as<std::string>();
            val.title = j.at("title").template as<std::string>();
            val.price = j.at("price").template as<double>();
            return val;
        }
        static Json to_json(const ns::book& val, 
                            allocator_type alloc=allocator_type())
        {
            Json j(alloc);
            j.try_emplace("author", val.author);
            j.try_emplace("title", val.title);
            j.try_emplace("price", val.price);
            return j;
        }
    };
} // namespace jsoncons
```

To save typing and enhance readability, the jsoncons library defines macros, 
so you could also write

```cpp
JSONCONS_ALL_MEMBER_TRAITS(ns::book, author, title, price)
```

which expands to the code above.

```cpp
using namespace jsoncons; // for convenience

int main()
{
    const std::string s = R"(
    [
        {
            "author" : "Haruki Murakami",
            "title" : "Kafka on the Shore",
            "price" : 25.17
        },
        {
            "author" : "Charles Bukowski",
            "title" : "Pulp",
            "price" : 22.48
        }
    ]
    )";

    std::vector<ns::book> book_list = decode_json<std::vector<ns::book>>(s);

    std::cout << "(1)\n";
    for (const auto& item : book_list)
    {
        std::cout << item.author << ", " 
                  << item.title << ", " 
                  << item.price << "\n";
    }

    std::cout << "\n(2)\n";
    encode_json(book_list, std::cout, indenting::indent);
    std::cout << "\n\n";
}
```
Output:
```
(1)
Haruki Murakami, Kafka on the Shore, 25.17
Charles Bukowski, Pulp, 22.48

(2)
[
    {
        "author": "Haruki Murakami",
        "price": 25.17,
        "title": "Kafka on the Shore"
    },
    {
        "author": "Charles Bukowski",
        "price": 22.48,
        "title": "Pulp"
    }
]
```


#### Specialize json_type_traits for a container type that the jsoncons library also supports

```cpp
#include <cassert>
#include <string>
#include <vector>
#include <jsoncons/json.hpp>

//own vector will always be of an even length 
struct own_vector : std::vector<int64_t> { using  std::vector<int64_t>::vector; };

namespace jsoncons {

template <typename Json>
struct json_type_traits<Json, own_vector> 
{
    static bool is(const Json& j) noexcept
    { 
        return j.is_object() && j.size() % 2 == 0;
    }
    static own_vector as(const Json& j)
    {   
        own_vector v;
        for (auto& item : j.object_range())
        {
            std::string s(item.key());
            v.push_back(std::strtol(s.c_str(),nullptr,10));
            v.push_back(item.value().template as<int64_t>());
        }
        return v;
    }
    static Json to_json(const own_vector& val){
                Json j;
                for(std::size_t i=0;i<val.size();i+=2){
                        j[std::to_string(val[i])] = val[i + 1];
                }
                return j;
        }
};

template <> 
struct is_json_type_traits_declared<own_vector> : public std::true_type 
{}; 
} // namespace jsoncons

using jsoncons::json;

int main()
{
    json j(json_object_arg, {{"1",2},{"3",4}});
    assert(j.is<own_vector>());
    auto v = j.as<own_vector>();
    json j2 = v;

    std::cout << j2 << "\n";
}
```
Output:
```
{"1":2,"3":4}
```


#### Convert JSON to/from boost matrix

```cpp
#include <jsoncons/json.hpp>
#include <boost/numeric/ublas/matrix.hpp>

namespace jsoncons {

    template <typename Json,typename T>
    struct json_type_traits<Json,boost::numeric::ublas::matrix<T>>
    {
        static bool is(const Json& val) noexcept
        {
            if (!val.is_array())
            {
                return false;
            }
            if (val.size() > 0)
            {
                std::size_t n = val[0].size();
                for (const auto& a: val.array_range())
                {
                    if (!(a.is_array() && a.size() == n))
                    {
                        return false;
                    }
                    for (auto x: a.array_range())
                    {
                        if (!x.template is<T>())
                        {
                            return false;
                        }
                    }
                }
            }
            return true;
        }

        static boost::numeric::ublas::matrix<T> as(const Json& val)
        {
            if (val.is_array() && val.size() > 0)
            {
                std::size_t m = val.size();
                std::size_t n = 0;
                for (const auto& a : val.array_range())
                {
                    if (a.size() > n)
                    {
                        n = a.size();
                    }
                }

                boost::numeric::ublas::matrix<T> A(m,n,T());
                for (std::size_t i = 0; i < m; ++i)
                {
                    const auto& a = val[i];
                    for (std::size_t j = 0; j < a.size(); ++j)
                    {
                        A(i,j) = a[j].template as<T>();
                    }
                }
                return A;
            }
            else
            {
                boost::numeric::ublas::matrix<T> A;
                return A;
            }
        }

        static Json to_json(const boost::numeric::ublas::matrix<T>& val)
        {
            Json a = Json::template make_array<2>(val.size1(), val.size2(), T());
            for (std::size_t i = 0; i < val.size1(); ++i)
            {
                for (std::size_t j = 0; j < val.size1(); ++j)
                {
                    a[i][j] = val(i,j);
                }
            }
            return a;
        }
    };
} // namespace jsoncons

using namespace jsoncons;
using boost::numeric::ublas::matrix;

int main()
{
    matrix<double> A(2, 2);
    A(0, 0) = 1.1;
    A(0, 1) = 2.1;
    A(1, 0) = 3.1;
    A(1, 1) = 4.1;

    json a = A;

    std::cout << "(1) " << std::boolalpha << a.is<matrix<double>>() << "\n\n";

    std::cout << "(2) " << std::boolalpha << a.is<matrix<int>>() << "\n\n";

    std::cout << "(3) \n\n" << pretty_print(a) << "\n\n";

    matrix<double> B = a.as<matrix<double>>();

    std::cout << "(4) \n\n";
    for (std::size_t i = 0; i < B.size1(); ++i)
    {
        for (std::size_t j = 0; j < B.size2(); ++j)
        {
            if (j > 0)
            {
                std::cout << ",";
            }
            std::cout << B(i, j);
        }
        std::cout << "\n";
    }
    std::cout << "\n\n";
}
```
Output:
```
(1) true

(2) false

(3)

[
    [1.1,2.1],
    [3.1,4.1]
]

(4)

1.1,2.1
3.1,4.1
``` 

