add void callbacks

README.md

@@ -302,6 +302,12 @@ env.add_callback("double-greetings", 0, [greet](Arguments args) {
 	return greet + " " + greet + "!";
 });
 env.render("{{ double-greetings }}", data); // "Hello Hello!"
+
+// You can also add a callback without return variable, e.g. for debugging:
+env.add_void_callback("log", 1, [greet](Arguments args) {
+	std::cout << "logging: " << args[0] << std::endl;
+});
+env.render("{{ log(neighbour) }}", data); // Prints nothing to result, only to cout...
 ```
 
 ### Comments

include/inja/environment.hpp

@@ -173,7 +173,14 @@ class Environment {
   @brief Adds a variadic callback
   */
   void add_callback(const std::string &name, const CallbackFunction &callback) {
-    function_storage.add_callback(name, -1, callback);
+    add_callback(name, -1, callback);
+  }
+
+  /*!
+  @brief Adds a variadic void callback
+  */
+  void add_void_callback(const std::string &name, const VoidCallbackFunction &callback) {
+    add_void_callback(name, -1, callback);
   }
 
   /*!
@@ -183,6 +190,13 @@ class Environment {
     function_storage.add_callback(name, num_args, callback);
   }
 
+  /*!
+  @brief Adds a void callback with given number or arguments
+  */
+  void add_void_callback(const std::string &name, int num_args, const VoidCallbackFunction &callback) {
+    function_storage.add_callback(name, num_args, [callback](Arguments& args) { callback(args); return json(); });
+  }
+
   /** Includes a template with a given name into the environment.
    * Then, a template can be rendered in another template using the
    * include "<name>" syntax.

include/inja/function_storage.hpp

@@ -13,6 +13,7 @@ using json = nlohmann::json;
 
 using Arguments = std::vector<const json *>;
 using CallbackFunction = std::function<json(Arguments &args)>;
+using VoidCallbackFunction = std::function<void(Arguments &args)>;
 
 /*!
  * \brief Class for builtin functions and user-defined callbacks.

include/inja/renderer.hpp

@@ -54,10 +54,9 @@ class Renderer : public NodeVisitor  {
   void print_json(const std::shared_ptr<json> value) {
     if (value->is_string()) {
       *output_stream << value->get_ref<const json::string_t&>();
-    } else if (value->is_number_float()) {
-      *output_stream << value->dump();
     } else if (value->is_number_integer()) {
       *output_stream << value->get<const json::number_integer_t>();
+    } else if (value->is_null()) {
     } else {
       *output_stream << value->dump();
     }

single_include/inja/inja.hpp

@@ -1526,6 +1526,7 @@ using json = nlohmann::json;
 
 using Arguments = std::vector<const json *>;
 using CallbackFunction = std::function<json(Arguments &args)>;
+using VoidCallbackFunction = std::function<void(Arguments &args)>;
 
 /*!
  * \brief Class for builtin functions and user-defined callbacks.
@@ -3348,10 +3349,9 @@ class Renderer : public NodeVisitor  {
   void print_json(const std::shared_ptr<json> value) {
     if (value->is_string()) {
       *output_stream << value->get_ref<const json::string_t&>();
-    } else if (value->is_number_float()) {
-      *output_stream << value->dump();
     } else if (value->is_number_integer()) {
       *output_stream << value->get<const json::number_integer_t>();
+    } else if (value->is_null()) {
     } else {
       *output_stream << value->dump();
     }
@@ -4061,7 +4061,14 @@ class Environment {
   @brief Adds a variadic callback
   */
   void add_callback(const std::string &name, const CallbackFunction &callback) {
-    function_storage.add_callback(name, -1, callback);
+    add_callback(name, -1, callback);
+  }
+
+  /*!
+  @brief Adds a variadic void callback
+  */
+  void add_void_callback(const std::string &name, const VoidCallbackFunction &callback) {
+    add_void_callback(name, -1, callback);
   }
 
   /*!
@@ -4071,6 +4078,13 @@ class Environment {
     function_storage.add_callback(name, num_args, callback);
   }
 
+  /*!
+  @brief Adds a void callback with given number or arguments
+  */
+  void add_void_callback(const std::string &name, int num_args, const VoidCallbackFunction &callback) {
+    function_storage.add_callback(name, num_args, [callback](Arguments& args) { callback(args); return json(); });
+  }
+
   /** Includes a template with a given name into the environment.
    * Then, a template can be rendered in another template using the
    * include "<name>" syntax.

test/test-functions.cpp

@@ -228,10 +228,15 @@ TEST_CASE("callbacks") {
     return number1.length();
   });
 
+  env.add_void_callback("log", 1, [](inja::Arguments args) {
+    int a = 2;
+  });
+
   env.add_callback("multiply", 0, [](inja::Arguments args) { return 1.0; });
 
   CHECK(env.render("{{ double(age) }}", data) == "56");
   CHECK(env.render("{{ half(age) }}", data) == "14");
+  CHECK(env.render("{{ log(age) }}", data) == "");
   CHECK(env.render("{{ double-greetings }}", data) == "Hello Hello!");
   CHECK(env.render("{{ double-greetings() }}", data) == "Hello Hello!");
   CHECK(env.render("{{ multiply(4, 5) }}", data) == "20.0");