implemented pretty printing (issue #13)

- to_string() method is now called dump()
- syntax borrowed from Python’s json.dumps()

README.md

@@ -112,13 +112,13 @@ You can create an object (deserialization) by appending `_json` to a string lite
 
 ```cpp
 // create object from string literal
-json j = "{ \"pi\": 3.141, \"happy\": true }"_json;
+json j = "{ \"happy\": true, \"pi\": 3.141 }"_json;
 
 // or even nicer (thanks http://isocpp.org/blog/2015/01/json-for-modern-cpp)
 auto j2 = R"(
   {
-    "pi": 3.141,
+    "happy": true,
-    "happy": true
+    "pi": 3.141
   })"_json;
 ```
 
@@ -126,7 +126,14 @@ You can also get a string representation (serialize):
 
 ```cpp
 // explicit conversion to string
-std::string s = j.to_string();
+std::string s = j.dump();    // {\"happy\": true, \"pi\": 3.141}
+
+// serialization with pretty printing
+std::cout << j.dump(4) << std::endl;
+// {
+//     "happy": true,
+//     "pi": 3.141
+// }
 ```
 
 The value of s could be `{"pi": 3.141, "happy": true}`, but the order of the entries in the object is not fixed.

src/json.cc

@@ -472,8 +472,22 @@ json::operator object_t() const
     return get<object_t>();
 }
 
-const std::string json::to_string() const noexcept
+/*!
+Internal implementation of the serialization function.
+
+\param prettyPrint    whether the output shall be pretty-printed
+\param indentStep     the indent level
+\param currentIndent  the current indent level (only used internally)
+*/
+const std::string json::dump(const bool prettyPrint,
+                             const unsigned int indentStep, unsigned int currentIndent) const noexcept
 {
+    // helper function to return whitespace as indentation
+    const auto indent = [prettyPrint, &currentIndent]()
+    {
+        return prettyPrint ? std::string(currentIndent, ' ') : std::string();
+    };
+
     switch (type_)
     {
         case (value_type::string):
@@ -498,34 +512,73 @@ const std::string json::to_string() const noexcept
 
         case (value_type::array):
         {
-            std::string result;
+            if (value_.array->empty())
+            {
+                return "[]";
+            }
+
+            std::string result = "[";
+
+            // increase indentation
+            if (prettyPrint)
+            {
+                currentIndent += indentStep;
+                result += "\n";
+            }
 
             for (array_t::const_iterator i = value_.array->begin(); i != value_.array->end(); ++i)
             {
                 if (i != value_.array->begin())
                 {
-                    result += ", ";
+                    result += prettyPrint ? ",\n" : ", ";
                 }
-                result += i->to_string();
+                result += indent() + i->dump(prettyPrint, indentStep, currentIndent);
             }
 
-            return "[" + result + "]";
+            // decrease indentation
+            if (prettyPrint)
+            {
+                currentIndent -= indentStep;
+                result += "\n";
+            }
+
+            return result + indent() + "]";
         }
 
         case (value_type::object):
         {
-            std::string result;
+            if (value_.object->empty())
+            {
+                return "{}";
+            }
+
+            std::string result = "{";
+
+            // increase indentation
+            if (prettyPrint)
+            {
+                currentIndent += indentStep;
+                result += "\n";
+            }
 
             for (object_t::const_iterator i = value_.object->begin(); i != value_.object->end(); ++i)
             {
                 if (i != value_.object->begin())
                 {
-                    result += ", ";
+                    result += prettyPrint ? ",\n" : ", ";
                 }
-                result += "\"" + i->first + "\": " + i->second.to_string();
+                result += indent() + "\"" + i->first + "\": " + i->second.dump(prettyPrint, indentStep,
+                          currentIndent);
             }
 
-            return "{" + result + "}";
+            // decrease indentation
+            if (prettyPrint)
+            {
+                currentIndent -= indentStep;
+                result += "\n";
+            }
+
+            return result + indent() + "}";
         }
 
         // actually only value_type::null - but making the compiler happy
@@ -536,6 +589,29 @@ const std::string json::to_string() const noexcept
     }
 }
 
+/*!
+Serialization function for JSON objects. The function tries to mimick Python's
+\p json.dumps() function, and currently supports its \p indent parameter.
+
+\param indent  if indent is nonnegative, then array elements and object members
+               will be pretty-printed with that indent level. An indent level
+               of 0 will only insert newlines. -1 (the default) selects the
+               most compact representation
+
+\see https://docs.python.org/2/library/json.html#json.dump
+*/
+const std::string json::dump(int indent) const noexcept
+{
+    if (indent >= 0)
+    {
+        return dump(true, static_cast<unsigned int>(indent));
+    }
+    else
+    {
+        return dump(false, 0);
+    }
+}
+
 
 ///////////////////////////////////////////
 // ADDING ELEMENTS TO OBJECTS AND ARRAYS //

src/json.h

@@ -162,6 +162,9 @@ class json
     /// return the type as string
     const std::string type_name() const noexcept;
 
+    /// dump the object (with pretty printer)
+    const std::string dump(const bool, const unsigned int, unsigned int = 0) const noexcept;
+
   public:
     /// explicit value conversion
     template<typename T>
@@ -180,34 +183,34 @@ class json
     /// implicit conversion to JSON map (only for objects)
     operator object_t() const;
 
-    /// write to stream
+    /// serialize to stream
     friend std::ostream& operator<<(std::ostream& o, const json& j)
     {
-        o << j.to_string();
+        o << j.dump();
         return o;
     }
-    /// write to stream
+    /// serialize to stream
     friend std::ostream& operator>>(const json& j, std::ostream& o)
     {
-        o << j.to_string();
+        o << j.dump();
         return o;
     }
 
-    /// read from stream
+    /// deserialize from stream
     friend std::istream& operator>>(std::istream& i, json& j)
     {
         j = parser(i).parse();
         return i;
     }
-    /// read from stream
+    /// deserialize from stream
     friend std::istream& operator<<(json& j, std::istream& i)
     {
         j = parser(i).parse();
         return i;
     }
 
-    /// explicit conversion to string representation (C++ style)
+    /// explicit serialization
-    const std::string to_string() const noexcept;
+    const std::string dump(int = -1) const noexcept;
 
     /// add an object/array to an array
     json& operator+=(const json&);

test/json_unit.cc

@@ -17,7 +17,7 @@ TEST_CASE("array")
         const json j_const (j);
 
         // string representation of default value
-        CHECK(j.to_string() == "[]");
+        CHECK(j.dump() == "[]");
 
         // iterators
         CHECK(j.begin() != j.end());
@@ -305,7 +305,7 @@ TEST_CASE("object")
         const json j_const = j;
 
         // string representation of default value
-        CHECK(j.to_string() == "{}");
+        CHECK(j.dump() == "{}");
 
         // iterators
         CHECK(j.begin() != j.end());
@@ -685,7 +685,7 @@ TEST_CASE("null")
         CHECK(j.type() == json::value_type::null);
 
         // string representation of default value
-        CHECK(j.to_string() == "null");
+        CHECK(j.dump() == "null");
 
         // iterators
         CHECK(j.begin() != j.end());
@@ -755,7 +755,7 @@ TEST_CASE("string")
         CHECK(j.cbegin() != j.cend());
 
         // string representation of default value
-        CHECK(j.to_string() == "\"\"");
+        CHECK(j.dump() == "\"\"");
 
         // container members
         CHECK(j.size() == 1);
@@ -837,7 +837,7 @@ TEST_CASE("boolean")
         CHECK(j.cbegin() != j.cend());
 
         // string representation of default value
-        CHECK(j.to_string() == "false");
+        CHECK(j.dump() == "false");
 
         // container members
         CHECK(j.size() == 1);
@@ -916,7 +916,7 @@ TEST_CASE("number (int)")
         CHECK(j.cbegin() != j.cend());
 
         // string representation of default value
-        CHECK(j.to_string() == "0");
+        CHECK(j.dump() == "0");
 
         // container members
         CHECK(j.size() == 1);
@@ -1002,7 +1002,7 @@ TEST_CASE("number (float)")
         CHECK(j.cbegin() != j.cend());
 
         // string representation of default value
-        CHECK(j.to_string() == "0.000000");
+        CHECK(j.dump() == "0.000000");
 
         // container members
         CHECK(j.size() == 1);
@@ -1786,6 +1786,18 @@ TEST_CASE("Parser")
         CHECK(j22 == j23);
     }
 
+    SECTION("serialization")
+    {
+        auto j23 = "{ \"a\": null, \"b\": true, \"c\": [1,2,3], \"d\": {\"a\": 0} }"_json;
+
+        CHECK(j23.dump() == "{\"a\": null, \"b\": true, \"c\": [1, 2, 3], \"d\": {\"a\": 0}}");
+        CHECK(j23.dump(-1) == "{\"a\": null, \"b\": true, \"c\": [1, 2, 3], \"d\": {\"a\": 0}}");
+        CHECK(j23.dump(0) ==
+              "{\n\"a\": null,\n\"b\": true,\n\"c\": [\n1,\n2,\n3\n],\n\"d\": {\n\"a\": 0\n}\n}");
+        CHECK(j23.dump(4) ==
+              "{\n    \"a\": null,\n    \"b\": true,\n    \"c\": [\n        1,\n        2,\n        3\n    ],\n    \"d\": {\n        \"a\": 0\n    }\n}");
+    }
+
     SECTION("Errors")
     {
         CHECK_THROWS_AS(json::parse(""), std::invalid_argument);