What is the Use of UpdateData(TRUE) and UpdateData(FALSE)?

In MFC (Microsoft Foundation Class), `UpdateData(TRUE)` and `UpdateData(FALSE)` are crucial functions for data exchange between controls and variables in dialog-based applications. This mechanism is known as DDX (Dynamic Data Exchange), and it allows you to synchronize user input in the UI with the underlying variables.

What Does UpdateData(TRUE) Do?

When you call `UpdateData(TRUE)`, MFC retrieves the data from the dialog controls and stores it into the corresponding member variables. This is particularly useful when you need to process the user input.

What Does UpdateData(FALSE) Do?

Calling `UpdateData(FALSE)` takes the data from the member variables and populates the controls in the dialog with that data. This is often used to initialize dialog controls or to update the UI based on changes in the code.

How to Use UpdateData in MFC

Consider the following example of a dialog with a text box and an integer variable:
“`cpp
class CMyDialog : public CDialog
{
// Data members
int m_nValue;

protected:
virtual void DoDataExchange(CDataExchange* pDX);
};

void CMyDialog::DoDataExchange(CDataExchange* pDX)
{
CDialog::DoDataExchange(pDX);
DDX_Text(pDX, IDC_EDIT1, m_nValue); // Synchronize the variable with the text box
}
“`

Example Usage of UpdateData(TRUE)

You would typically call `UpdateData(TRUE)` when you want to retrieve user input, such as when a button is pressed:
“`cpp
void CMyDialog::OnOK()
{
if (UpdateData(TRUE))
{
// Process the value entered by the user
AfxMessageBox(_T(“User entered value: “) + std::to_string(m_nValue).c_str());
}
}
“`

Example Usage of UpdateData(FALSE)

Call `UpdateData(FALSE)` to update the controls based on variable changes:
“`cpp
void CMyDialog::InitializeDialog()
{
m_nValue = 100; // Initialize value
UpdateData(FALSE); // Update the UI
}
“`

Understanding `UpdateData(TRUE)` and `UpdateData(FALSE)` is essential for synchronizing data in MFC applications.