worked on #91

- implemented `get_ptr` function to return pointer to value member
- overworked `get` function to support pointer types
- added test cases
- added documentation (see
http://nlohmann.github.io/json/classnlohmann_1_1basic__json.html) with
examples

doc/examples/get__PointerType.cpp

@@ -0,0 +1,20 @@
+#include <json.hpp>
+
+using namespace nlohmann;
+
+int main()
+{
+    // create a JSON boolean
+    json value = 17;
+
+    // explicitly getting pointers
+    auto p1 = value.get<const json::number_integer_t*>();
+    auto p2 = value.get<json::number_integer_t*>();
+    auto p3 = value.get<json::number_integer_t* const>();
+    auto p4 = value.get<const json::number_integer_t* const>();
+    auto p5 = value.get<json::number_float_t*>();
+
+    // print the pointees
+    std::cout << *p1 << ' ' << *p2 << ' ' << *p3 << ' ' << *p4 << '\n';
+    std::cout << std::boolalpha << (p5 == nullptr) << '\n';
+}

doc/examples/get__PointerType.output

@@ -0,0 +1,2 @@
+17 17 17 17
+true

doc/examples/get__ValueType_const.cpp

@@ -0,0 +1,49 @@
+#include <json.hpp>
+#include <unordered_map>
+
+using namespace nlohmann;
+
+int main()
+{
+    // create a JSON value with different types
+    json json_types =
+    {
+        {"boolean", true},
+        {
+            "number", {
+                {"integer", 42},
+                {"floating-point", 17.23}
+            }
+        },
+        {"string", "Hello, world!"},
+        {"array", {1, 2, 3, 4, 5}},
+        {"null", nullptr}
+    };
+
+    // use explicit conversions
+    auto v1 = json_types["boolean"].get<bool>();
+    auto v2 = json_types["number"]["integer"].get<int>();
+    auto v3 = json_types["number"]["integer"].get<short>();
+    auto v4 = json_types["number"]["floating-point"].get<float>();
+    auto v5 = json_types["number"]["floating-point"].get<int>();
+    auto v6 = json_types["string"].get<std::string>();
+    auto v7 = json_types["array"].get<std::vector<short>>();
+    auto v8 = json_types.get<std::unordered_map<std::string, json>>();
+
+    // print the conversion results
+    std::cout << v1 << '\n';
+    std::cout << v2 << ' ' << v3 << '\n';
+    std::cout << v4 << ' ' << v5 << '\n';
+    std::cout << v6 << '\n';
+
+    for (auto i : v7)
+    {
+        std::cout << i << ' ';
+    }
+    std::cout << "\n\n";
+
+    for (auto i : v8)
+    {
+        std::cout << i.first << ": " << i.second << '\n';
+    }
+}

doc/examples/get__ValueType_const.output

@@ -0,0 +1,11 @@
+1
+42 42
+17.23 17
+Hello, world!
+1 2 3 4 5 
+
+string: "Hello, world!"
+number: {"floating-point":17.23,"integer":42}
+null: null
+boolean: true
+array: [1,2,3,4,5]

doc/examples/get_ptr.cpp

@@ -0,0 +1,20 @@
+#include <json.hpp>
+
+using namespace nlohmann;
+
+int main()
+{
+    // create a JSON boolean
+    json value = 17;
+
+    // explicitly getting pointers
+    auto p1 = value.get_ptr<const json::number_integer_t*>();
+    auto p2 = value.get_ptr<json::number_integer_t*>();
+    auto p3 = value.get_ptr<json::number_integer_t* const>();
+    auto p4 = value.get_ptr<const json::number_integer_t* const>();
+    auto p5 = value.get_ptr<json::number_float_t*>();
+
+    // print the pointees
+    std::cout << *p1 << ' ' << *p2 << ' ' << *p3 << ' ' << *p4 << '\n';
+    std::cout << std::boolalpha << (p5 == nullptr) << '\n';
+}

doc/examples/get_ptr.output

@@ -0,0 +1,2 @@
+17 17 17 17
+true

doc/examples/operator__ValueType.cpp

@@ -0,0 +1,49 @@
+#include <json.hpp>
+#include <unordered_map>
+
+using namespace nlohmann;
+
+int main()
+{
+    // create a JSON value with different types
+    json json_types =
+    {
+        {"boolean", true},
+        {
+            "number", {
+                {"integer", 42},
+                {"floating-point", 17.23}
+            }
+        },
+        {"string", "Hello, world!"},
+        {"array", {1, 2, 3, 4, 5}},
+        {"null", nullptr}
+    };
+
+    // use implicit conversions
+    bool v1 = json_types["boolean"];
+    int v2 = json_types["number"]["integer"];
+    short v3 = json_types["number"]["integer"];
+    float v4 = json_types["number"]["floating-point"];
+    int v5 = json_types["number"]["floating-point"];
+    std::string v6 = json_types["string"];
+    std::vector<short> v7 = json_types["array"];
+    std::unordered_map<std::string, json> v8 = json_types;
+
+    // print the conversion results
+    std::cout << v1 << '\n';
+    std::cout << v2 << ' ' << v3 << '\n';
+    std::cout << v4 << ' ' << v5 << '\n';
+    std::cout << v6 << '\n';
+
+    for (auto i : v7)
+    {
+        std::cout << i << ' ';
+    }
+    std::cout << "\n\n";
+
+    for (auto i : v8)
+    {
+        std::cout << i.first << ": " << i.second << '\n';
+    }
+}

doc/examples/operator__ValueType.output

@@ -0,0 +1,11 @@
+1
+42 42
+17.23 17
+Hello, world!
+1 2 3 4 5 
+
+string: "Hello, world!"
+number: {"floating-point":17.23,"integer":42}
+null: null
+boolean: true
+array: [1,2,3,4,5]