Add front methods to json_pointers (implements #4889) (#5152)

* Added front, pop_front, and push_front methods to json_pointers in order to facilitate root-to-leaf traversals of JSON object trees. (#4889)

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* undid VS autoformatting in irrelevant code

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* Ran make amalgamate, added navigation to json_pointer's new front methods in mkdocs, and fixed errors in documented complexity for those methods.

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* Fixed GCC 4.8 compile error caused by const iterators

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* Fixed another gcc-4.8 compile error

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* amalgamated

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

---------

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

docs/mkdocs/docs/examples/json_pointer__front.cpp

@@ -0,0 +1,15 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // different JSON Pointers
+    json::json_pointer ptr1("/foo");
+    json::json_pointer ptr2("/foo/0");
+
+    // call empty()
+    std::cout << "first reference token of \"" << ptr1 << "\" is \"" << ptr1.front() << "\"\n"
+              << "first reference token of \"" << ptr2 << "\" is \"" << ptr2.front() << "\"" << std::endl;
+}

docs/mkdocs/docs/examples/json_pointer__front.output

@@ -0,0 +1,2 @@
+first reference token of "/foo" is "foo"
+first reference token of "/foo/0" is "foo"

docs/mkdocs/docs/examples/json_pointer__pop_front.cpp

@@ -0,0 +1,21 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create empty JSON Pointer
+    json::json_pointer ptr("/foo/bar/baz");
+    std::cout << "\"" << ptr << "\"\n";
+
+    // call pop_front()
+    ptr.pop_front();
+    std::cout << "\"" << ptr << "\"\n";
+
+    ptr.pop_front();
+    std::cout << "\"" << ptr << "\"\n";
+
+    ptr.pop_front();
+    std::cout << "\"" << ptr << "\"\n";
+}

docs/mkdocs/docs/examples/json_pointer__pop_front.output

@@ -0,0 +1,4 @@
+"/foo/bar/baz"
+"/bar/baz"
+"/baz"
+""

docs/mkdocs/docs/examples/json_pointer__push_front.cpp

@@ -0,0 +1,21 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create empty JSON Pointer
+    json::json_pointer ptr;
+    std::cout << "\"" << ptr << "\"\n";
+
+    // call push_front()
+    ptr.push_front("foo");
+    std::cout << "\"" << ptr << "\"\n";
+
+    ptr.push_front("0");
+    std::cout << "\"" << ptr << "\"\n";
+
+    ptr.push_front("bar");
+    std::cout << "\"" << ptr << "\"\n";
+}

docs/mkdocs/docs/examples/json_pointer__push_front.output

@@ -0,0 +1,4 @@
+""
+"/foo"
+"/0/foo"
+"/bar/0/foo"