Passing reference parameter to return the value?

Question

I've got a program that is to be written using three different methods, one with a return type, one without a return type one with a reference parameter to return the value.

The first two are set up like this:

private: System::Void btnConvert_Click(System::Object^  sender, System::EventArgs^  e) {
// [Programmer-defined method WITHOUT a return type]
				 //double amount;
				 //Double::TryParse(txtAmount->Text, amount);
				 //ConvertIt(amount);
				 
// [Programmer-defined method WITH a return type]
                 //double amount,answer;
		//Double::TryParse(txtAmount->Text, amount);
		//answer = ConvertIt(amount);
		//txtAnswer->Text = answer.ToString();

I'm not understanding the 3rd method which is to use a method with a reference parameter to return
the value:

The book has it set up like this:

private: System::Void btnConvert_Click(System::Object^  sender, System::EventArgs^  e) {
// [Programmer-defined method with a reference parameterto return the value]

				 double amount, answer;
				 Double::TryParse(txtAmount->Text, amount);
				 ConvertIt(amount,answer);
				 txtAnswer->Text = answer.ToString();

Previously, I was just passing amount to the method where [answer] could be calculated. I don't understand why answer is being passed when it hasn't been calculated?

Answer 1

The 2nd parameter to double::TryParse isn't really a reference parameter, it's an out parameter. This has similar semantics to a reference parameter, but different expectations on usage. Such a thing isn't formalized in C++, only in C++/CLI (and even then, not to the extent that it is in C#).

Answer 2

Sorry, I'm not understanding. I'm only parsing and then passing the amount since the answer depends on the amount to be calculated.

When I used the programmer method without a return type. I was just passing the amount. Like the code below:

private: System::Void btnConvert_Click(System::Object^  sender, System::EventArgs^  e) {

// [Programmer-defined method WITHOUT a return type]
//double amount;
//Double::TryParse(txtAmount->Text, amount);
//ConvertIt(amount);

I was then passing amount to the function ConvertIt, which is below:

 private: void ConvertIt(double dis_amount)
//{

// [Programmer-defined method WITHOUT a return type]
//private: void ConvertIt(double dis_amount)
//{
//double answer;

//if (cboFrom->Text == "millimeter" && cboTo->Text == "inch")
//	 answer = dis_amount / 25.4;

I don't really understand setting up the function with this reference parameter to return a value when answer is one of the parameters being passed. I can understand it being passed to a display or output function but I don't understand how they are using it here. I know this must be a good logical reason for setting it up the way they are asking but I don't understand it.

Answer 3

I don't really understand setting up the function with this reference parameter to return a value when answer is one of the parameters being passed. I can understand it being passed to a display or output function but I don't understand how they are using it here. I know this must be a good logical reason for setting it up the way they are asking but I don't understand it.

I have no clue what your question is. answer isn't being passed anywhere in the code you've shown in this post.