From 9225cf2f572df13027e67bda5b188ae6d2702605 Mon Sep 17 00:00:00 2001
From: Patrick Boettcher <p@yai.se>
Date: Tue, 15 Jan 2019 14:39:06 +0100
Subject: [PATCH] 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 | 11 ++++-
 single_include/nlohmann/json.hpp         | 11 ++++-
 test/src/unit-json_pointer.cpp           | 54 ++++++++++++++++++++++++
 3 files changed, 74 insertions(+), 2 deletions(-)

diff --git a/include/nlohmann/detail/json_pointer.hpp b/include/nlohmann/detail/json_pointer.hpp
index 3521b889db..033625bc4c 100644
--- a/include/nlohmann/detail/json_pointer.hpp
+++ b/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
     {
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 70b6c8a855..e3b54c88f6 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -11885,7 +11885,6 @@ class json_pointer
         return res;
     }
 
-  private:
     /*!
     @brief remove and return last reference pointer
     @throw out_of_range.405 if JSON pointer has no parent
@@ -11902,6 +11901,16 @@ 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
     {
diff --git a/test/src/unit-json_pointer.cpp b/test/src/unit-json_pointer.cpp
index ac9cd71e2a..1bcd3938a7 100644
--- a/test/src/unit-json_pointer.cpp
+++ b/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");
+    }
 }