add simple format-callback-example

Addresses #129

CMakeLists.txt

@@ -155,6 +155,9 @@ if (BUILD_EXAMPLES)
     add_executable(readme-json-schema app/readme.cpp)
     target_link_libraries(readme-json-schema nlohmann_json_schema_validator)
 
+    add_executable(format-json-schema app/format.cpp)
+    target_link_libraries(format-json-schema nlohmann_json_schema_validator)
+
     install(TARGETS json-schema-validate readme-json-schema
             DESTINATION bin)
 endif()

app/format.cpp

@@ -0,0 +1,54 @@
+#include <iostream>
+
+#include <nlohmann/json-schema.hpp>
+
+using nlohmann::json;
+using nlohmann::json_schema::json_validator;
+
+// The schema is defined based upon a string literal
+static json uri_schema = R"(
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "type": "object",
+  "properties": {
+    "myUri": {
+      "type":"string",
+      "format": "uri"
+    }
+  }
+})"_json;
+
+// The people are defined with brace initialization
+static json good_uri = {{"myUri", "http://hostname.com/"}};
+static json bad_uri = {{"myUri", "http:/hostname.com/"}};
+
+static void uri_format_checker(const std::string &format, const std::string &value)
+{
+	if (format == "uri") {
+		if (value.find("://") == std::string::npos)
+			throw std::invalid_argument("URI does not contain :// - invalid");
+	} else
+		throw std::logic_error("Don't know how to validate " + format);
+}
+
+int main()
+{
+	json_validator validator(nullptr, uri_format_checker); // create validator
+
+	try {
+		validator.set_root_schema(uri_schema); // insert root-schema
+	} catch (const std::exception &e) {
+		std::cerr << "Validation of schema failed, here is why: " << e.what() << "\n";
+		return EXIT_FAILURE;
+	}
+
+	validator.validate(good_uri);
+
+	try {
+		validator.validate(bad_uri);
+	} catch (const std::exception &e) {
+		std::cerr << "Validation expectedly failed, here is why: " << e.what() << "\n";
+	}
+
+	return EXIT_SUCCESS;
+}