Return types, pass by reference vs pass by value (#20261)

Expanded on what data types can be returned. Also added section for the differences between passing an object as a parameter by value versus by reference.

guide/english/cplusplus/functions/index.md

@@ -19,7 +19,15 @@ return_type function_name( parameter list )
 ```
 
 ### Return type:
-A function may return a value. The `return_type` is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the `return_type` is the keyword `void`, but the [return type of main() must always be int](https://stackoverflow.com/a/4207223/).
+A function may return a value. The `return_type` is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the `return_type` is the keyword `void`, but the [return type of main() must always be int](https://stackoverflow.com/a/4207223/). In C++, the return type of a function can be any primitive data type, a reference to an object, or a pointer to an object.
+```cpp
+int func1();      // Returns a primitive data type
+char & func2();   // Returns a reference to an object (Careful not to return a obeject local to the function)
+float * func3();  // Returns a pointers to a float
+
+struct node{};
+node * func4();  // Returns a pointer to a user defined data type
+```
 
 ### Function name:
 This is the actual name of the function. The function name and the parameter list together constitute the function signature.
@@ -27,6 +35,36 @@ This is the actual name of the function. The function name and the parameter lis
 ### Parameters:
 A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
 
+The parameters of a function are passed by value unless explicitly specified by the programmer. This means that the program will create a temporary copy of the arguments, and any changes made to those copies from within the function will not remain once the function returns.
+
+The address of operator, `&`, allows the programmer to pass in a parameter by reference. When the function is invoked, the program will not create a copy of the parameter, and any changes made from within the function will remain once the function returns.
+
+```cpp
+// Function prototypes
+void increment_by_value (int a); 
+void increment_by_reference (int & a);
+
+int main() {
+   int number = 0;
+   
+   // Invoking functions
+   increment_by_value(number);   // The value of number stays at 0
+   increment_by_reference(number);  // The value of number changes to 1
+   return 1;
+}
+
+// Function definitions
+void increment_by_value (int a) {
+   a = a + 1;
+   return;
+}
+
+void increment_by_reference (int & a) {
+   a = a + 1;
+   return;
+}
+```
+
 ### Function body:
 The function body contains a collection of statements that define what the function does.