allow push_back() and pop_back() calls on json_pointer

Putting pop_back() to public and creating a trivial push_back()
method allows users of nlohmann::json_pointer to manipulate an
existing json-pointer by adding or removing keys at the end.

This is useful for traversing a JSON-instance and keeping track
of its "absolute path" at any moment.

In my case for a schema-validator error-handler.

include/nlohmann/detail/json_pointer.hpp

@@ -97,7 +97,6 @@ class json_pointer
         return res;
     }
 
-  private:
     /*!
     @brief remove and return last reference pointer
     @throw out_of_range.405 if JSON pointer has no parent
@@ -114,6 +113,16 @@ class json_pointer
         return last;
     }
 
+    /*!
+    @brief remove and return last reference pointer
+    @throw out_of_range.405 if JSON pointer has no parent
+    */
+    void push_back(const std::string &tok)
+    {
+        reference_tokens.push_back(tok);
+    }
+
+  private:
     /// return whether pointer points to the root document
     bool is_root() const noexcept
     {

test/src/unit-json_pointer.cpp

@@ -459,4 +459,58 @@ TEST_CASE("JSON pointers")
             CHECK(j.is_object());
         }
     }
+
+	SECTION("push and pop")
+	{
+        const json j =
+        {
+			{"", "Hello"},
+            {"pi", 3.141},
+            {"happy", true},
+            {"name", "Niels"},
+            {"nothing", nullptr},
+            {
+                "answer", {
+                    {"everything", 42}
+                }
+            },
+            {"list", {1, 0, 2}},
+            {
+                "object", {
+                    {"currency", "USD"},
+                    {"value", 42.99},
+                    {"", "empty string"},
+                    {"/", "slash"},
+                    {"~", "tilde"},
+                    {"~1", "tilde1"}
+                }
+            }
+        };
+
+		// empty json_pointer returns the root JSON-object
+		auto ptr = ""_json_pointer;
+		CHECK(j[ptr] == j);
+
+		// simple field access
+		ptr.push_back("pi");
+		CHECK(j[ptr] == j["pi"]);
+
+		ptr.pop_back();
+		CHECK(j[ptr] == j);
+
+		// object and children access
+		ptr.push_back("answer");
+		ptr.push_back("everything");
+		CHECK(j[ptr] == j["answer"]["everything"]);
+
+		ptr.pop_back();
+		ptr.pop_back();
+		CHECK(j[ptr] == j);
+
+		// push key which has to be encoded
+		ptr.push_back("object");
+		ptr.push_back("/");
+		CHECK(j[ptr] == j["object"]["/"]);
+		CHECK(ptr.to_string() == "/object/~1");
+	}
 }