MSVC/MFC File Processing: MFC's Standard I/O File Processing

	Microsoft Visual C++/MFC File Processing: MFC's Standard I/O File Processing

Introduction

The MFC library provides it own version of C's file processing. This is done through a class named CStdioFile. The CStdioFile class is derived from CFile.

As done for the FILE structure, to start normal file input/output operations, declare a variable of type CStdioFile. This class has five constructors whose syntaxes are:

CStdioFile();
CStdioFile(CAtlTransactionManager* pTM);
CStdioFile(FILE* pOpenStream);
CStdioFile(LPCTSTR lpszFileName, UINT nOpenFlags);
CStdioFile(LPCTSTR lpszFileName, UINT nOpenFlags, CAtlTransactionManager* pTM);

The default constructor allows you to initiate file processing without giving much detail. If you use it, you can then call the Open() method that the CStdioFile class inherits from CFile. Pass the same arguments you would for a CFile variable.

Practical Learning: Introducing MFC's Standard File Processing

To create a new application, on the main menu, click File -> New Project...
In the middle list, click MFC Application
Set the Name to LoanPreparation1
Click OK
In the first page of the MFC Application Wizard, click Next
In the second page of the wizard, click Dialog Box
Click Next
In the third page of the wizard, change the Dialog Title to Loan Preparation
Click Next twice
Click Finish
On the dialog box, click TODO: and press Delete
Press the OK button and press Delete
Click the Cancel button to select it
In the Properties window, click Caption, type Close and press Enter

Design the dialog box as follows:

Control	Caption	ID	Right Align Text
Static Text	Prepared By:
Edit Control		IDC_CUSTOMERNAME
Static Text	Prepared For:
Edit Control		IDC_EMPLOYEENAME
Static Text	________________
Static Text	Loan Amount:
Edit Control		IDC_PRINCIPAL	True
Static Text	Interest Rate:
Edit Control		IDC_INTERESTRATE	True
Static Text	%
Static Text	Paid In:
Edit Control		IDC_PERIODS	True
Static Text	Months
Button	Evaluate	IDC_EVALUATE
Static Text	_________________
Static Text	Future Value:
Edit Control		IDC_FUTUREVALUE	True
Static Text	Monthly Payment:
Edit Control		IDC_MONTHLYPAYMENT	True
Static Text	_________________
Static Text	Save As:
Edit Control		IDC_FILESAVE	True
Button	Save	IDC_SAVE_BTN
Static Text	File to Open:
Edit Control		IDC_FILEOPEN	True
Button	Open	IDC_OPEN_BTN
Static Text	File Name:
Button	Reset	IDC_RESET_BTN
Button	Close	IDCANCEL

Right-click each edit control, click Add Variable, select the category as Value, set the options as follows and click Finish on each:

Control ID	Category	Variable Type	Variable Name
IDC_CUSTOMERNAME	Value	CString	m_CustomerName
IDC_EMPLOYEENAME	Value	CString	m_EmployeeName
IDC_PRINCIPAL	Value	CString	m_Principal
IDC_INTERESTRATE	Value	CString	m_InterestRate
IDC_PERIODS	Value	CString	m_Periods
IDC_FUTUREVALUE	Value	CString	m_FutureValue
IDC_MONTHLYPAYMENT	Value	CString	m_MonthlyPayment
IDC_FILESAVE	Value	CString	m_FileSave
IDC_FILEOPEN	Value	CString	m_FileOpen

Access the source file of the dialog box and change the initializations in the constructor as follows:

CLoanPreparation1Dlg::CLoanPreparation1Dlg(CWnd* pParent /*=NULL*/)
	: CDialogEx(CLoanPreparation1Dlg::IDD, pParent)
	, m_CustomerName(_T(""))
	, m_EmployeeName(_T(""))
	, m_Principal(_T("0.00"))
	, m_InterestRate(_T("0.00"))
	, m_Periods(_T("0"))
	, m_FutureValue(_T("0.00"))
	, m_MonthlyPayment(_T("0.00"))
	, m_FileSave(_T(""))
	, m_FileOpen(_T(""))
{
	m_hIcon = AfxGetApp()->LoadIcon(IDR_MAINFRAME);
}

Return to the dialog box
Double-click the Evaluate button

Implement the event as follows:

void CLoanPreparation1Dlg::OnBnClickedEvaluate()
{
	UpdateData(TRUE);

	double Principal      = _wtof(m_Principal);
	double InterestRate   = _wtof(m_InterestRate) / 100;
	double Periods	      = _wtof(m_Periods) / 12;
	double InterestAmount = Principal * InterestRate * Periods;
	double FutureValue    = Principal + InterestAmount;
	double MonthlyPayment = FutureValue / _wtof(m_Periods);
	
	m_FutureValue.Format(L"%.2f", FutureValue);
	m_MonthlyPayment.Format(L"%.2f", MonthlyPayment);

	UpdateData(FALSE);
}

Return to the dialog box
Double-click the Reset button

Implement the event as follows:

void CLoanPreparation1Dlg::OnBnClickedResetBtn()
{
	m_CustomerName 	 = L"";
	m_EmployeeName 	 = L"";
	m_Principal    	 = L"0.00";
	m_InterestRate 	 = L"0.00";
	m_Periods      	 = L"0.00";
	m_FutureValue  	 = L"0.00";
	m_MonthlyPayment = L"0.00";
	m_FileSave 	 = L"";
	m_FileOpen 	 = L"";

	UpdateData(FALSE);
}

Writing to Standard Stream

Naturally the primary operation you would want to perform on a file consists of writing one or more values to it. To let you continually write a string without any concern for the end of line, you can use the Write() method that the CStdioFile gets from its parent. If you want the compiler to add a new line at the end of each written string, you can use the WriteString() method of the CStdioFile class. Its syntax is:

virtual void WriteString(LPCTSTR lpsz);

Practical Learning: Writing Values to a Stream

On the dialog box, double-click the Save button

Implement its event as follows:

void CLoanPreparation1Dlg::OnBnClickedSaveBtn()
{
	CStdioFile sioLoan;
	UpdateData(TRUE);
	
	if( m_EmployeeName.GetLength() == 0 )
	{
		AfxMessageBox(L"You must specify the name of the employee who prepared this loan.");
		return;
	}
	
	if( m_CustomerName.GetLength() == 0 )
	{
		AfxMessageBox(L"You must specify the name of the customer for whom the loan was prepared.");
		return;
	}
	
	if( m_FileSave.GetLength() == 0 )
	{
		AfxMessageBox(L"You must enter a name for the file to save.");
		return;
	}

	sioLoan.Open(m_FileSave + L".txt", CFile::modeCreate | CFile::modeWrite | CFile::typeText);

	sioLoan.WriteString(m_EmployeeName);
	sioLoan.WriteString(L"\n");
	sioLoan.WriteString(m_CustomerName);
	sioLoan.WriteString(L"\n");
	sioLoan.WriteString(m_Principal);
	sioLoan.WriteString(L"\n");
	sioLoan.WriteString(m_InterestRate);
	sioLoan.WriteString(L"\n");
	sioLoan.WriteString(m_Periods);
	sioLoan.WriteString(L"\n");
	sioLoan.WriteString(m_FutureValue);
	sioLoan.WriteString(L"\n");
	sioLoan.WriteString(m_MonthlyPayment);

	sioLoan.Close();

	OnBnClickedResetBtn();
	UpdateData(FALSE);
}

Execute the application to test
Enter the Prepared By value as Ernestine Stanley
Enter the Prepared For name as Benjamin Greens
Enter the Loan Amount as 5000.00
Enter the Interest Rate as 12.25
Enter the Paid In value as 36
Click Evaluate
In the Save As edit control, type BG as the name of the file
Click Save
Close the dialog box and return to your programming environment

Reading From a Standard Stream

To read a continuous string from a stream, you can call the Read() method that the CStdioFile class gets from its parent. If you have a document made of various paragraphs separated by new lines, to let you read each line, the CStdioFile class is equipped with the ReadString() method. It comes in two versions whose syntaxes are:

virtual LPTSTR ReadString(LPTSTR lpsz, UINT nMax);
virtual BOOL ReadString(CString& rString);

The first version is used to write a C string to the file and you must specify the number of characters you are writing, as the second argument. The second version can be more convenient as it accepts a CString value.

Practical Learning: Reading Values From a Stream

On the dialog box, double-click the Open button

Implement its event as follows:

void CLoanPreparation1Dlg::OnBnClickedOpenBtn()
{
	UpdateData(TRUE);
	
	if( m_FileOpen.GetLength() == 0 )
	{
		AfxMessageBox(L"You must enter a name for the file to open.");
		return;
	}

	CStdioFile sioLoan;
	sioLoan.Open(m_FileOpen + L".txt", CFile::modeRead | CFile::typeText);

	sioLoan.ReadString(m_EmployeeName);
	sioLoan.ReadString(m_CustomerName);
	sioLoan.ReadString(m_Principal);
	sioLoan.ReadString(m_InterestRate);
	sioLoan.ReadString(m_Periods);
	sioLoan.ReadString(m_FutureValue);
	sioLoan.ReadString(m_MonthlyPayment);

	UpdateData(FALSE);
	sioLoan.Close();
}

Execute the application to test the dialog box
In the File to Open edit control, enter the name of the file you previously created (BG) and click Open
Close the dialog box and return to your programming environment