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storage_ptr

Variable-length containers in this library all use dynamically allocated memory to store their contents. Callers can gain control over the strategy used for allocation by specifying a storage_ptr in select constructors and function parameter lists. A storage_ptr has these properties:

This lists all of the allocation-related types and functions available when using the library:

Table 1.7. Functions and Types

Name

Description

get_null_resource

Returns a pointer to a memory resource instance which always throws an exception upon allocation. This is used to to achieve the invariant that no parsing or container operation will dynamically allocate memory.

is_deallocate_trivial

A customization point allowing a memory resource type to indicate that calls to deallocate are trivial.

make_shared_resource

A function returning a smart pointer with shared ownership of a newly allocated memory resource.

memory_resource

The abstract base class representing an allocator.

monotonic_resource

A memory resource which allocates large blocks of memory and has a trivial deallocate function. Allocated memory is not freed until the resource is destroyed, making it fast for parsing but not suited for performing modifications.

polymorphic_allocator

An Allocator which uses a reference to a memory_resource to perform allocations.

static_resource

A memory resource that uses a single caller provided buffer. No dynamic allocations are used. This is fast for parsing but not suited for performing modifications.

storage_ptr

A smart pointer through which a memory_resource is managed and accessed.


Default Memory Resource

The default memory resource wraps calls to the global heap allocation functions. This resource is not reference counted and has a non-trivial deallocate function. All default-constructed storage_ptr reference the same default memory resource:

storage_ptr sp1;
storage_ptr sp2;

assert( sp1.get() != nullptr );                         // always points to a valid resource
assert( sp1.get() == sp2.get() );                       // both point to the default resource
assert( *sp1.get() == *sp2.get() );                     // the default resource compares equal

Default-constructed library containers use the default memory resource:

array arr;                                              // default construction
object obj;
string str;
value jv;

assert( jv.storage().get() == storage_ptr().get() );    // uses the default memory resource
assert( jv.storage().get() == arr.storage().get() );    // both point to the default resource
assert( *arr.storage() == *obj.storage() );             // containers use equivalent resources

The default memory resource is well suited for general usage. It offers reasonable performance for parsing, and conservative memory usage for modification of the contents of containers.

Monotonic Resource

Consider the pattern of memory allocation during parsing: when an array, object, or string is encountered the parser accumulates elements in its temporary storage area. When all of the elements are known, a single memory allocation is requested from the resource when constructing the value. Thus, parsing only allocates and constructs containers at their final size. Memory is not reallocated; that is, a memory buffer never needs to grow by allocating a new larger buffer and deallocating the previous buffer.

The monotonic_resource optimizes this memory allocation pattern by allocating increasingly large blocks of global memory internally and parceling those blocks out in smaller pieces to fulfill allocation requests. It has a trivial deallocate function. The monotonic resource does not actually deallocate memory until the resource is destroyed. Thus, it is ideally suited for the use-case where JSON is parsed, and the resulting value is then inspected but not modified.

The resource to use when constructing values may be specified in calls to parse as shown here:

monotonic_resource mr;

value const jv = parse( "[1,2,3]", &mr );

Or, to parse into a value with shared ownership of the memory resource:

value parse_value( string_view s)
{
    return parse( s, make_shared_resource< monotonic_resource >() );
}

A monotonic resource may be optionally constructed with an initial buffer to use first, before going to the heap. This allows the caller to use stack space and avoid dynamic allocations for most parsed JSON, falling back to dynamic allocation from the heap if the input JSON is larger than average, as shown here:

template< class Handler >
void do_rpc( string_view s, Handler&& h )
{
    unsigned char buffer[ 8192 ];                       // Small stack buffer to avoid most allocations during parse
    monotonic_resource mr( buffer );                    // This resource will use our local buffer first
    value const jv = parse( s, &mr );                   // Parse the input string into a value that uses our resource
    h( jv );                                            // Call the handler to perform the RPC command
}
Static Resource

A static_resource constructs from a caller-provided buffer, and satisfies all memory allocation requests from the buffer. Once the buffer is exhausted, subsequent calls to allocate throw the exception std::bad_alloc. The resource offers a simple invariant: dynamic heap allocations are never performed.

To use the resource, construct it with a local buffer:

unsigned char buffer[ 8192 ];
static_resource mr( buffer );                           // The resource will use our local buffer
Null Resource

The function get_null_resource returns a global instance of the null resource. This resource offers a simple invariant: all calls to allocate will throw the exception std::bad_alloc. An instance of the null resource can be used to make parsing guarantee that allocations from the heap are never made. This is explored in more detail in a later section.

Allocator Propagation

The containers array, object, and value all propagate the memory resource they were constructed with to child elements:

monotonic_resource mr;
array arr( &mr );                                       // construct an array using our resource
arr.emplace_back( "boost" );                            // insert a string
assert( *arr[0].as_string().storage() == mr );          // the resource is propagated to the string

This propagation acts recursively, containers within containers will all have the resource propagated. Once a container is constructed, its memory resource can never be changed.

Resource Lifetime

It is important to note that storage_ptr supports both shared-ownership and reference lifetime models. Construction from a memory resource pointer does not transfer ownership:

{
    monotonic_resource mr;

    array arr( &mr );                                   // construct an array using our resource

    assert( ! arr.storage().is_shared() );              // no shared ownership
}

When using a memory resource in this fashion, including the case where a storage pointer or container is constructed from a polymorphic_allocator, the caller must ensure that the lifetime of the resource is extended until it is no longer referenced by any variables; otherwise, undefined behavior is possible.

Shared ownership is achieved using the function make_shared_resource, which creates a new, reference-counted memory resource using a dynamic memory allocation and returns it as a storage_ptr:

storage_ptr sp = make_shared_resource< monotonic_resource >();

string str( sp );

assert( sp.is_shared() );                               // shared ownership
assert( str.storage().is_shared() );                    // shared ownership

When a storage pointer is constructed this way, the lifetime of the referenced memory resource is extended until all variables which reference it are destroyed.

User-Defined Resource

To implement custom memory allocation strategies, derive your class from memory_resource and implement the functions do_allocate, do_deallocate, and do_is_equal as seen in this example below, which logs each operation it performs to the console:

class logging_resource : public memory_resource
{
private:
    void* do_allocate( std::size_t bytes, std::size_t align ) override
    {
        std::cout << "Allocating " << bytes << " bytes with alignment " << align << '\n';

        return ::operator new( bytes );
    }

    void do_deallocate( void* ptr, std::size_t bytes, std::size_t align ) override
    {
        std::cout << "Deallocating " << bytes << " bytes with alignment " << align << " @ address " << ptr << '\n';

        return ::operator delete( ptr );
    }

    bool do_is_equal( memory_resource const& other ) const noexcept override
    {
        // since the global allocation and deallocation functions are used,
        // any instance of a logging_resource can deallocate memory allocated
        // by another instance of a logging_resource

        return dynamic_cast< logging_resource const* >( &other ) != nullptr;
    }
};

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