Making Your C++ Code Robust(让你的c++代码变得更强大)

Introduction

When your C++ program grows in size, it becomes difficult to keep track of its components, and it's easier to introduce bugs. So it is important to realize that some rules should be followed to write code more tolerant to errors.

This article provides some tips that may help you to write code more tolerant to critical errors (crashes). Even if your program currently doesn't follow any coding rules because of its complexity and because of your project's team structure, following the simple rules listed below may help you avoid the majority of crash situations.

Background

For some softwares I develop, I have been using an Open-Source error reporting library called CrashRpt for a long time. The CrashRpt library allows to automatically submit error reports from installations of my software on users' computers. This way all critical errors (exceptions) in my software are automatically reported to me via the Internet, allowing me to improve my software and make users happier with each hot fix release.

Figure 1 - Error report dialog of CrashRpt library

While analyzing incoming error reports, I found that often the reason for program crashes can be easily avoided by following simple rules. For example, sometimes I forget to initialize a local variable and it might contain trash causing an array index overflow. Sometimes I don't perform a NULL check before using a pointer variable and this causes an access violation.

I have summarised my experience in several coding rules below that may help you to avoid such mistakes and make your software more stable (robust).

Initializing Local Variables

Non-initialized local variables are a common reason for program crashes. For example, see the following code fragment:

// Define local variables BOOL bExitResult; // This will be TRUE if the function exits successfully FILE* f; // Handle to file TCHAR szBuffer[_MAX_PATH]; // String buffer // Do something with variables above...

The code fragment above can be a potential reason for a crash, because non of the local variables are initialized. When your code runs, these variables will contain some trash values. For example, the bExitResult boolean variable may contain the value -135913245 (not boolean by nature). Or the szBuffer string variable may not be zero-terminated as it must be. So initializing local variables is very important.

The correct code would be the following:

// Define local variables // Initialize function exit code with FALSE to indicate failure assumption BOOL bExitResult = FALSE; // This will be TRUE if the function exits successfully // Initialize file handle with NULL FILE* f = NULL; // Handle to file // Initialize string buffer with empty string TCHAR szBuffer[_MAX_PATH] = _T(""); // String buffer // Do something with variables above...

Note: One may say that for some time-critical calculations, variable initialization may be costly, and they are right. If your code should execute as rapidly as possible, you may skip variable initialization at your own risk.

Initializing WinAPI Structures

Many WinAPI functions receive/return parameters through C structures. Such a structure, if incorrectly initialized, may be the reason for a crash. It is recommended to use the ZeroMemory() macro or the memset() function to fill structures with zeroes (this typically sets the structure fields to their default values).

Many WinAPI structures also have the cbSize parameter that must be initialized with the size of the structure before using.

The following code shows how to initialize a WinAPI structure:

NOTIFYICONDATA nf; // WinAPI structure memset(&nf,0,sizeof(NOTIFYICONDATA)); // Zero memory nf.cbSize = sizeof(NOTIFYICONDATA); // Set structure size! // Initialize other structure members nf.hWnd = hWndParent; nf.uID = 0; nf.uFlags = NIF_ICON | NIF_TIP; nf.hIcon = ::LoadIcon(NULL, IDI_APPLICATION); _tcscpy_s(nf.szTip, 128, _T("Popup Tip Text")); // Add a tray icon Shell_NotifyIcon(NIM_ADD, &nf);

But! Do not use ZeroMemory() or memset() for your C++ structures that contain objects as structure members; that may corrupt their internal state and be the reason for a crash.But! Do not use ZeroMemory() or memset() for your C++ structures that contain objects as structure members; that may corrupt their internal state and be the reason for a crash.But! Do not use ZeroMemory() or memset() for your C++ structures that contain objects as structure members; that may corrupt their internal state and be the reason for a crash.But! Do not use ZeroMemory() or memset() for your C++ structures that contain objects as structure members; that may corrupt their internal state and be the reason for a crash.

// Declare a C++ structure struct ItemInfo { // The structure has std::string object inside std::string sItemName; int nItemValue; }; // Init the structure ItemInfo item; // Do not use memset()! It can corrupt the structure // memset(&item, 0, sizeof(ItemInfo)); // Instead use the following item.sItemName = "item1"; item.nItemValue = 0;

It is even better to use a constructor for your C++ structure that would init its members with default values:

// Declare a C++ structure struct ItemInfo { // Use structure constructor to set members with default values ItemInfo() { sItemName = _T("unknown"); nItemValue = -1; } std::string sItemName; // The structure has std::string object inside int nItemValue; }; // Init the structure ItemInfo item; // Do not use memset()! It can corrupt the structure // memset(&item, 0, sizeof(ItemInfo)); // Instead use the following item.sItemName = "item1"; item.nItemValue = 0;

Validating Function Input

It is recommended to always validate function input parameters. For example, if your function is a part of the public API of a dynamic link library, and it may be called by an external client, it is not guaranteed that the external client will pass you the correct parameters.

For example, let's look at the hypothetical DrawVehicle() function that draws a sports car on a window with a varying quality. The drawing quality nDrawingQaulity may vary from 0 (coarse quality) to 100 (fine quality). The drawing rectangle in prcDraw defines where to draw the car.

Below is the code. Note how we validate the input parameter values before using them.

BOOL DrawVehicle(HWND hWnd, LPRECT prcDraw, int nDrawingQuality) { // Check that window is valid if(!IsWindow(hWnd)) return FALSE; // Check that drawing rect is valid if(prcDraw==NULL) return FALSE; // Check drawing quality is valid if(nDrawingQuality<0 || nDrawingQuality>100) return FALSE; // Now it's safe to draw the vehicle // ... return TRUE; }

Validating Pointers

Using a pointer without validation is very common. I would even say this is the main reason for crashes in my software.

If you use a pointer, make sure it is not equal to NULL. If your code tries to use a NULL pointer, this may result in an access violation exception.

CVehicle* pVehicle = GetCurrentVehicle(); // Validate pointer if(pVehicle==NULL) { // Invalid pointer, do not use it! return FALSE; } // Use the pointer

Initializing Function Output

If your function creates an object and returns it as a function parameter, it is recommended to initialize the pointer with NULL in the beginning of the function body.

If you do not explicitly initialize the output parameter and further it is not set due to a bug in function logics, the caller may use the invalid pointer which would possibly cause a crash.

Here is an example of incorrect code:

int CreateVehicle(CVehicle** ppVehicle) { if(CanCreateVehicle()) { *ppVehicle = new CVehicle(); return 1; } // If CanCreateVehicle() returns FALSE, // the pointer to *ppVehcile would never be set! return 0; }

The correct code:

int CreateVehicle(CVehicle** ppVehicle) { // First initialize the output parameter with NULL *ppVehicle = NULL; if(CanCreateVehicle()) { *ppVehicle = new CVehicle(); return 1; } return 0; }

Cleaning Up Pointers to Deleted Objects

Assign NULL to a pointer after freeing (or deleting) it. This will help to ensure no one will try to reuse an invalid pointer. As you may suppose, accessing a pointer to a deleted object results in an access violation exception.

The following code example shows how to clean up a pointer to a deleted object.

// Create object CVehicle* pVehicle = new CVehicle(); delete pVehicle; // Free pointer pVehicle = NULL; // Set pointer with NULL

Cleaning Up Released Handles

Assign NULL (or zero, or some default value) to a handle after freeing it. This will help ensure no one will try to reuse an invalid handle.

Below is an example of how to clean up a WinAPI file handle:

HANDLE hFile = INVALID_HANDLE_VALUE; // Open file hFile = CreateFile(_T("example.dat"), FILE_READ|FILE_WRITE, FILE_OPEN_EXISTING); if(hFile==INVALID_HANDLE_VALUE) { return FALSE; // Error opening file } // Do something with file // Finally, close the handle if(hFile!=INVALID_HANDLE_VALUE) { CloseHandle(hFile); // Close handle to file hFile = INVALID_HANDLE_VALUE; // Clean up handle <span style="font-size: 16px;">}</span>

Below is an example of how to clean up a FILE* handle:

// First init file handle pointer with NULL FILE* f = NULL; // Open handle to file errno_t err = _tfopen_s(_T("example.dat"), _T("rb")); if(err!=0 || f==NULL) return FALSE; // Error opening file // Do something with file // When finished, close the handle if(f!=NULL) // Check that handle is valid { fclose(f); f = NULL; // Clean up pointer to handle }

Using the delete [] Operator for Arrays

If you allocate a single object with the operator new, you should free it with the operator delete.

But if you allocate an array of objects with the operator new, you should free this array with delete [].

// Create an array of objects CVehicle* paVehicles = new CVehicle[10]; delete [] paVehicles; // Free pointer to array paVehicles = NULL; // Set pointer with NULL

or:

// Create a buffer of bytes LPBYTE pBuffer = new BYTE[255]; delete [] pBuffer; // Free pointer to array pBuffer = NULL; // Set pointer with NULL Allocating Memory Carefully

Allocating Memory Carefully

Sometimes it is required to allocate a buffer dynamically, but the buffer size is determined at run time. For example, if you need to read a file into memory, you allocate a memory buffer whose size is equal to the size of the file. But before allocating the buffer, ensure that 0 (zero) bytes are not allocated using malloc() or new. Passing the incorrect parameter to malloc() results in a C run-time error.

The following code example shows dynamic buffer allocation:

// Determine what buffer to allocate. UINT uBufferSize = GetBufferSize(); LPBYTE* pBuffer = NULL; // Init pointer to buffer // Allocate a buffer only if buffer size > 0 if(uBufferSize>0) pBuffer = new BYTE[uBufferSize];

For additional tips on how to allocate memory correctly, you can read the Secure Coding Best Practices for Memory Allocation in C and C++ article.

Using Asserts Carefully

Asserts can be used in debug mode for checking pre-conditions and post-conditions. But when you compile your program in Release mode, asserts are removed on the pre-processing stage. So, using asserts is not enough to validate your program's state.

Incorrect code:

#include <assert.h> // This function reads a sports car's model from a file CVehicle* ReadVehicleModelFromFile(LPCTSTR szFileName) { CVehicle* pVehicle = NULL; // Pointer to vehicle object // Check preconditions assert(szFileName!=NULL); // This will be removed by preprocessor in Release mode! assert(_tcslen(szFileName)!=0); // This will be removed in Release mode! // Open the file FILE* f = _tfopen(szFileName, _T("rt")); // Create new CVehicle object pVehicle = new CVehicle(); // Read vehicle model from file // Check postcondition assert(pVehicle->GetWheelCount()==4); // This will be removed in Release mode! // Return pointer to the vehicle object return pVehicle; }

As you can see in the code above, usage of asserts can help you to check your program state in Debug mode, but in Release mod,e these checks will just disappear. So, in addition to asserts, you have to use if() checks.

The correct code would be:

#include <assert.h> CVehicle* ReadVehicleModelFromFile(LPCTSTR szFileName, ) { CVehicle* pVehicle = NULL; // Pointer to vehicle object // Check preconditions assert(szFileName!=NULL); // This will be removed by preprocessor in Release mode! assert(_tcslen(szFileName)!=0); // This will be removed in Release mode! if(szFileName==NULL || _tcslen(szFileName)==0) return NULL; // Invalid input parameter // Open the file FILE* f = _tfopen(szFileName, _T("rt")); // Create new CVehicle object pVehicle = new CVehicle(); // Read vehicle model from file // Check postcondition assert(pVehicle->GetWheelCount()==4); // This will be removed in Release mode! if(pVehicle->GetWheelCount()!=4) { // Oops... an invalid wheel count was encountered! delete pVehicle; pVehicle = NULL; } // Return pointer to the vehicle object return pVehicle; <span style="font-size: 18px;">}</span>

Checking Return Code of a Function

It is a common mistake to call the function and assume it will succeed. When you call a function, it is recommended to check its return code and values of output parameters.

The following code calls functions in succession. Whether to proceed or to exit depends on return code and output parameters.

HRESULT hres = E_FAIL; IWbemServices *pSvc = NULL; IWbemLocator *pLoc = NULL; hres = CoInitializeSecurity( NULL, -1, // COM authentication NULL, // Authentication services NULL, // Reserved RPC_C_AUTHN_LEVEL_DEFAULT, // Default authentication RPC_C_IMP_LEVEL_IMPERSONATE, // Default Impersonation NULL, // Authentication info EOAC_NONE, // Additional capabilities NULL // Reserved ); if (FAILED(hres)) { // Failed to initialize security if(hres!=RPC_E_TOO_LATE) return FALSE; } hres = CoCreateInstance( CLSID_WbemLocator, 0, CLSCTX_INPROC_SERVER, IID_IWbemLocator, (LPVOID *) &pLoc); if (FAILED(hres) || !pLoc) { // Failed to create IWbemLocator object. return FALSE; } hres = pLoc->ConnectServer( _bstr_t(L"ROOT\\CIMV2"), // Object path of WMI namespace NULL, // User name. NULL = current user NULL, // User password. NULL = current 0, // Locale. NULL indicates current NULL, // Security flags. 0, // Authority (e.g. Kerberos) 0, // Context object &pSvc // pointer to IWbemServices proxy ); if (FAILED(hres) || !pSvc) { // Couldn't conect server if(pLoc) pLoc->Release(); return FALSE; } hres = CoSetProxyBlanket( pSvc, // Indicates the proxy to set RPC_C_AUTHN_WINNT, // RPC_C_AUTHN_xxx RPC_C_AUTHZ_NONE, // RPC_C_AUTHZ_xxx NULL, // Server principal name RPC_C_AUTHN_LEVEL_CALL, // RPC_C_AUTHN_LEVEL_xxx RPC_C_IMP_LEVEL_IMPERSONATE, // RPC_C_IMP_LEVEL_xxx NULL, // client identity EOAC_NONE // proxy capabilities ); if (FAILED(hres)) { // Could not set proxy blanket. if(pSvc) pSvc->Release(); if(pLoc) pLoc->Release(); return FALSE; }

Using Smart Pointers

If you intensively use pointers to shared objects (e.g., COM interfaces), it is a good practice to wrap them into smart pointers. The smart pointer will take care of your object's reference counting and will protect you from accessing an object that was already deleted. That is, you don't need to worry about controlling the lifetime of your interface pointer.

For additional info on smart pointers, see the following articles: Smart Pointers - What, Why, Which? and Implementing a Simple Smart Pointer in C++.

Below is an example code (borrowed from MSDN) that uses ATL's CComPtr template class as a smart pointer.

#include <windows.h> #include <shobjidl.h> #include <atlbase.h> // Contains the declaration of CComPtr. int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE, PWSTR pCmdLine, int nCmdShow) { HRESULT hr = CoInitializeEx(NULL, COINIT_APARTMENTTHREADED | COINIT_DISABLE_OLE1DDE); if (SUCCEEDED(hr)) { CComPtr<IFileOpenDialog> pFileOpen; // Create the FileOpenDialog object. hr = pFileOpen.CoCreateInstance(__uuidof(FileOpenDialog)); if (SUCCEEDED(hr)) { // Show the Open dialog box. hr = pFileOpen->Show(NULL); // Get the file name from the dialog box. if (SUCCEEDED(hr)) { CComPtr<IShellItem> pItem; hr = pFileOpen->GetResult(&pItem); if (SUCCEEDED(hr)) { PWSTR pszFilePath; hr = pItem->GetDisplayName(SIGDN_FILESYSPATH, &pszFilePath); // Display the file name to the user. if (SUCCEEDED(hr)) { MessageBox(NULL, pszFilePath, L"File Path", MB_OK); CoTaskMemFree(pszFilePath); } } // pItem goes out of scope. } // pFileOpen goes out of scope. } CoUninitialize(); } return 0; }

Using == Operator Carefully

Look at the following code fragment:

CVehicle* pVehicle = GetCurrentVehicle(); // Validate pointer if(pVehicle==NULL) // Using == operator to compare pointer with NULL return FALSE; // Do something with the pointer pVehicle->Run();

The code above is correct and uses pointer validation. But assume you made a typing mistake and used an assignment operator (=) instead of the equality operator (==):

CVehicle* pVehicle = GetCurrentVehicle(); // Validate pointer if(pVehicle=NULL) // Oops! A mistyping here! return FALSE; // Do something with the pointer pVehicle->Run(); // Crash!!!

As you can see from the code above, such mistyping may be the result of a stupid crash.

Such an error can be avoided by slightly modifying the pointer validation code (exchange left side and right side of the equality operator). If you mistype in such a modified code, it will be detected on compilation stage.

// Validate pointer if(NULL==pVehicle) // Exchange left side and right side of the equality operator return FALSE; // Validate pointer if(NULL=pVehicle) // Oops! A mistyping here! But the compiler returns an error message. return FALSE;

link:http://www.codeproject.com/KB/exception/MakingYourCodeRobust.aspx