STL allocators are objects that encapsulate the logic and interface for allocating and deallocating memory for STL containers. They are templates that take a type parameter, which specifies the type of elements that the allocator can handle. Allocators provide methods such as allocate, deallocate, construct, and destroy, which are used by the containers to manage their elements. Allocators also define some types and constants, such as pointer, size_type, and max_size, which are used by the containers to interact with the allocator.
C++11 smart pointers are classes that wrap raw pointers and provide automatic memory management. They are templates that take a type parameter, which specifies the type of object that the pointer points to. Smart pointers implement the RAII (Resource Acquisition Is Initialization) idiom, which means that they acquire a resource in their constructor and release it in their destructor. Smart pointers also overload operators such as *, ->, and =, which allow them to behave like raw pointers. There are three main types of smart pointers in C++11: unique_ptr, shared_ptr, and weak_ptr.
STL allocators and C++11 smart pointers differ in several ways. First, they have different purposes and scopes. Allocators are meant to be used by STL containers to allocate and deallocate memory for their elements, while smart pointers are meant to be used by programmers to manage the lifetime of individual objects. Allocators are usually hidden from the user, while smart pointers are explicitly declared and manipulated. Second, they have different interfaces and semantics. Allocators provide low-level methods that operate on raw memory, while smart pointers provide high-level methods that operate on objects. Allocators do not keep track of the ownership or reference count of the memory they allocate, while smart pointers do. Allocators can be customized and replaced by the user, while smart pointers are predefined and fixed by the standard.
STL allocators and C++11 smart pointers resemble in some ways. First, they both use templates to achieve genericity and type safety. They both take a type parameter that specifies the type of memory or object they handle, and they both enforce compile-time checks and type conversions. Second, they both use RAII to ensure proper memory management. They both acquire memory in their constructors and release it in their destructors, avoiding memory leaks and dangling pointers. Third, they both support move semantics, which allow them to transfer ownership and resources without copying or destroying them. They both implement move constructors and move assignment operators, which enable efficient and expressive code.
STL allocators and C++11 smart pointers are useful in different scenarios and contexts. You should use STL allocators when you need to customize or optimize the memory allocation and deallocation strategy for STL containers. For example, you can use a custom allocator to allocate memory from a pool, a stack, a shared memory segment, or a memory-mapped file. You can also use a custom allocator to implement debugging or profiling features, such as tracking memory usage or detecting memory errors. You should use C++11 smart pointers when you need to manage the lifetime and ownership of individual objects that are not stored in STL containers. For example, you can use a unique_ptr to express exclusive ownership of a dynamically allocated object, a shared_ptr to express shared ownership of a dynamically allocated object, or a weak_ptr to express non-owning references to a shared object.
STL allocators and C++11 smart pointers can work together to achieve more flexible and robust memory management. You can use STL containers with custom allocators to store smart pointers as elements, which allows you to control both the allocation and deallocation of the container and its elements. You can also use smart pointers with custom deleters to store pointers to objects that are allocated by STL allocators, which allows you to use RAII and polymorphism with allocator-managed objects. However, you should be careful to avoid mixing different allocators or smart pointers with incompatible types or semantics, which can lead to undefined behavior or memory errors.