add default function

README.md

@@ -182,6 +182,10 @@ render("{{ round(3.1415, 3) }}", data); // 3.142
 render("{{ odd(42) }}", data); // false
 render("{{ even(42) }}", data); // true
 render("{{ divisibleBy(42, 7) }}", data); // true
+
+// Set default values if variables are not defined
+render("Hello {{ default(neighbour, "my friend") }}!") // "Hello Peter!"
+render("Hello {{ default(colleague, "my friend") }}!") // "Hello my friend!"
 ```
 
 ### Callbacks

src/inja.hpp

@@ -250,6 +250,7 @@ struct Parsed {
 		Odd,
 		Even,
 		ReadJson,
+		Default,
 		Callback
 	};
 
@@ -443,6 +444,13 @@ public:
 				if (result.is_null()) { throw std::runtime_error("Did not found json element: " + element.command); }
 				return result;
 			}
+			case Parsed::Function::Default: {
+				try {
+					return eval_expression(element.args[0], data);
+				} catch (std::exception e) {
+					return eval_expression(element.args[1], data);
+				}
+			}
 			case Parsed::Function::Callback: {
 				return map_callbacks[element.command](element.args, data);
 			}
@@ -575,7 +583,8 @@ public:
 		{Parsed::Function::DivisibleBy, function_regex("divisibleBy", 2)},
 		{Parsed::Function::Odd, function_regex("odd", 1)},
 		{Parsed::Function::Even, function_regex("even", 1)},
-		{Parsed::Function::ReadJson, Regex{"\\s*([^\\(\\)]*\\S)\\s*"}}
+		{Parsed::Function::ReadJson, Regex{"\\s*([^\\(\\)]*\\S)\\s*"}},
+		{Parsed::Function::Default, function_regex("default", 2)}
 	};
 
 	std::map<std::string, Regex> regex_map_callbacks;

test/src/unit-renderer.cpp

@@ -133,6 +133,15 @@ TEST_CASE("functions") {
 		CHECK( env.render("{{ even(12) }}", data) == "true" );
 		// CHECK_THROWS_WITH( env.render("{{ even(name) }}", data), "[json.exception.type_error.302] type must be number, but is string" );
 	}
+
+	SECTION("default") {
+		CHECK( env.render("{{ default(11, 0) }}", data) == "11" );
+		CHECK( env.render("{{ default(nothing, 0) }}", data) == "0" );
+		CHECK( env.render("{{ default(name, \"nobody\") }}", data) == "Peter" );
+		CHECK( env.render("{{ default(surname, \"nobody\") }}", data) == "nobody" );
+
+		CHECK_THROWS_WITH( env.render("{{ default(surname, lastname) }}", data), "Did not found json element: lastname" );
+	}
 }
 
 TEST_CASE("callbacks") {