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Memory management in the the AWS SDK for C++ - AWS SDK for C++
Memory management in the the AWS SDK for C++ - AWS SDK for C++
Allocating and Deallocating MemorySTL and AWS Strings and VectorsRemaining IssuesNative SDK Developers and Memory Controls

Memory management in the the AWS SDK for C++

The AWS SDK for C++ provides a way to control memory allocation and deallocation in a library.

Note

Custom memory management is available only if you use a version of the library built using the defined compile-time constant USE_AWS_MEMORY_MANAGEMENT.

If you use a version of the library that is built without the compile-time constant, global memory system functions such as InitializeAWSMemorySystem won’t work; the global new and delete functions are used instead.

For more information about the compile-time constant, see STL and AWS Strings and Vectors.

Allocating and Deallocating Memory

To allocate or deallocate memory

  1. Subclass MemorySystemInterface: aws/core/utils/memory/MemorySystemInterface.h.

    class MyMemoryManager : public Aws::Utils::Memory::MemorySystemInterface
{
public:
    // ...
    virtual void* AllocateMemory(
        std::size_t blockSize, std::size_t alignment,
        const char *allocationTag = nullptr) override;
    virtual void FreeMemory(void* memoryPtr) override;
};
    Note

    You can change the type signature for AllocateMemory as needed.

  2. Use the Aws::SDKOptions struct to configure the use of the custom memory manager. Pass the instance of the struct into Aws::InitAPI. Before the application terminates, the SDK must be shut down by calling Aws::ShutdownAPI with the same instance. 

    int main(void)
{
  MyMemoryManager sdkMemoryManager;
  SDKOptions options;
  options.memoryManagementOptions.memoryManager = &sdkMemoryManager;
  Aws::InitAPI(options);

  // ... do stuff

  Aws::ShutdownAPI(options);

  return 0;
}

STL and AWS Strings and Vectors

When initialized with a memory manager, the AWS SDK for C++ defers all allocation and deallocation to the memory manager. If a memory manager doesn’t exist, the SDK uses global new and delete.

If you use custom STL allocators, you must alter the type signatures for all STL objects to match the allocation policy. Because STL is used prominently in the SDK implementation and interface, a single approach in the SDK would inhibit direct passing of default STL objects into the SDK or control of STL allocation. Alternately, a hybrid approach—using custom allocators internally and allowing standard and custom STL objects on the interface—could potentially make it more difficult to investigate memory issues.

The solution is to use the memory system’s compile-time constant USE_AWS_MEMORY_MANAGEMENT to control which STL types the SDK uses.

If the compile-time constant is enabled (on), the types resolve to STL types with a custom allocator connected to the AWS memory system.

If the compile-time constant is disabled (off), all Aws::* types resolve to the corresponding default std::* type.

Example code from the AWSAllocator.h file in the SDK 

#ifdef USE_AWS_MEMORY_MANAGEMENT

template< typename T >
class AwsAllocator : public std::allocator< T >
{
   ... definition of allocator that uses AWS memory system
};

#else

template< typename T > using Allocator = std::allocator<T>;

#endif

In the example code, the AwsAllocator can be a custom allocator or a default allocator, depending on the compile-time constant.

Example code from the AWSVector.h file in the SDK 

template<typename T> using Vector = std::vector<T, Aws::Allocator<T>>;

In the example code, we define the Aws::* types.

If the compile-time constant is enabled (on), the type maps to a vector using custom memory allocation and the AWS memory system.

If the compile-time constant is disabled (off), the type maps to a regular std::vector with default type parameters.

Type aliasing is used for all std:: types in the SDK that perform memory allocation, such as containers, string streams, and string buffers. The AWS SDK for C++ uses these types.

Remaining Issues

You can control memory allocation in the SDK; however, STL types still dominate the public interface through string parameters to the model object initialize and set methods. If you don’t use STL and use strings and containers instead, you have to create a lot of temporaries whenever you want to make a service call.

To remove most of the temporaries and allocation when you make service calls using non-STL, we have implemented the following:

  • Every Init/Set function that takes a string has an overload that takes a const char*.

  • Every Init/Set function that takes a container (map/vector) has an add variant that takes a single entry.

  • Every Init/Set function that takes binary data has an overload that takes a pointer to the data and a length value.

  • (Optional) Every Init/Set function that takes a string has an overload that takes a non-zero terminated const char* and a length value.

Native SDK Developers and Memory Controls

Follow these rules in the SDK code:

  • Don’t use new and delete; use Aws::New<> and Aws::Delete<> instead.

  • Don’t use new[] and delete[]; use Aws::NewArray<> and Aws::DeleteArray<>.

  • Don’t use std::make_shared; use Aws::MakeShared.

  • Use Aws::UniquePtr for unique pointers to a single object. Use the Aws::MakeUnique function to create the unique pointer.

  • Use Aws::UniqueArray for unique pointers to an array of objects. Use the Aws::MakeUniqueArray function to create the unique pointer.

  • Don’t directly use STL containers; use one of the Aws:: typedefs or add a typedef for the container you want. For example:

    Aws::Map<Aws::String, Aws::String> m_kvPairs;

  • Use shared_ptr for any external pointer passed into and managed by the SDK. You must initialize the shared pointer with a destruction policy that matches how the object was allocated. You can use a raw pointer if the SDK is not expected to clean up the pointer.