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
  {

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
  {

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");
+ }
 }