In modern C++ programming, the Standard Library includes smart pointers, which are used to help ensure that programs are free of memory and resource leaks and are exception-safe.
Smart pointers are defined in the std namespace in the <memory> header file. They are crucial to the RAII or Resource Acquisition Is Initialialization programming idiom. The main goal of this idiom is to ensure that resource acquisition occurs at the same time that the object is initialized, so that all resources for the object are created and made ready in one line of code. In practical terms, the main principle of RAII is to give ownership of any heap-allocated resource—for example, dynamically-allocated memory or system object handles—to a stack-allocated object whose destructor contains the code to delete or free the resource and also any associated cleanup code.
In most cases, when you initialize a raw pointer or resource handle to point to an actual resource, pass the pointer to a smart pointer immediately. In modern C++, raw pointers are only used in small code blocks of limited scope, loops, or helper functions where performance is critical and there is no chance of confusion about ownership.
The following example compares a raw pointer declaration to a smart pointer declaration.
void UseRawPointer() { // Using a raw pointer -- not recommended. Song* pSong = new Song(L"Nothing on You", L"Bruno Mars"); // Use pSong... // Don't forget to delete! delete pSong; } void UseSmartPointer() { // Declare a smart pointer on stack and pass it the raw pointer. unique_ptr<Song> song2(new Song(L"Nothing on You", L"Bruno Mars")); // Use song2... wstring s = song2->duration_; //... } // song2 is deleted automatically here.
As shown in the example, a smart pointer is a class template that you declare on the stack, and initialize by using a raw pointer that points to a heap-allocated object. After the smart pointer is initialized, it owns the raw pointer. This means that the smart pointer is responsible for deleting the memory that the raw pointer specifies. The smart pointer destructor contains the call to delete, and because the smart pointer is declared on the stack, its destructor is invoked when the smart pointer goes out of scope, even if an exception is thrown somewhere further up the stack.
Access the encapsulated pointer by using the familiar pointer operators, -> and *, which the smart pointer class overloads to return the encapsulated raw pointer.
The C++ smart pointer idiom resembles object creation in languages such as C#: you create the object and then let the system take care of deleting it at the correct time. The difference is that no separate garbage collector runs in the background; memory is managed through the standard C++ scoping rules so that the runtime environment is faster and more efficient.
Important
|
|---|
|
Always create smart pointers on a separate line of code, never in a parameter list, so that a subtle resource leak won't occur due to certain parameter list allocation rules. |
The following example shows how a unique_ptr smart pointer type from the Standard Template Library could be used to encapsulate a pointer to a large object.
class LargeObject { public: void DoSomething(){} }; void ProcessLargeObject(const LargeObject& lo){} void SmartPointerDemo() { // Create the object and pass it to a smart pointer std::unique_ptr<LargeObject> pLarge(new LargeObject()); //Call a method on the object pLarge->DoSomething(); // Pass a reference to a method. ProcessLargeObject(*pLarge); } //pLarge is deleted automatically when function block goes out of scope.
The example demonstrates the following essential steps for using smart pointers.
-
Declare the smart pointer as an automatic (local) variable. (Do not use the new or malloc expression on the smart pointer itself.)
-
In the type parameter, specify the pointed-to type of the encapsulated pointer.
-
Pass a raw pointer to a new-ed object in the smart pointer constructor. (Some utility functions or smart pointer constructors do this for you.)
-
Use the overloaded -> and * operators to access the object.
-
Let the smart pointer delete the object.
Smart pointers are designed to be as efficient as possible both in terms of memory and performance. For example, the only data member in unique_ptr is the encapsulated pointer. This means that unique_ptr is exactly the same size as that pointer, either four bytes or eight bytes. Accessing the encapsulated pointer by using the smart pointer overloaded * and -> operators is not significantly slower than accessing the raw pointers directly.
Smart pointers have their own member functions, which are accessed by using “dot” notation. For example, some STL smart pointers have a reset member function that releases ownership of the pointer. This is useful when you want to free the memory owned by the smart pointer before the smart pointer goes out of scope, as shown in the following example.
void SmartPointerDemo2() { // Create the object and pass it to a smart pointer std::unique_ptr<LargeObject> pLarge(new LargeObject()); //Call a method on the object pLarge->DoSomething(); // Free the memory before we exit function block. pLarge.reset(); // Do some other work... }
Smart pointers usually provide a way to access their raw pointer directly. STL smart pointers have a get member function for this purpose, and CComPtr has a public p class member. By providing direct access to the underlying pointer, you can use the smart pointer to manage memory in your own code and still pass the raw pointer to code that does not support smart pointers.
void SmartPointerDemo4() { // Create the object and pass it to a smart pointer std::unique_ptr<LargeObject> pLarge(new LargeObject()); //Call a method on the object pLarge->DoSomething(); // Pass raw pointer to a legacy API LegacyLargeObjectFunction(pLarge.get()); }
