Memory leak in face of exceptions

[ColinH]

The constructor often does a two-step initialisation, an allocate() followed by a construct(). If the construct() throws then the previously allocated memory will leak...

        AllocatorType<object_t> alloc;
        object = alloc.allocate(1);
        alloc.construct(object, value);

[gregmarr]

Probably the least intrusive way is to use a temporary unique_ptr with a custom deleter:

    AllocatorType<object_t> alloc;
    auto deleter = [&](object_t *object) { alloc.deallocate(object, 1); };
    std::unique_ptr<object_t, decltype(deleter)> local_object(alloc.allocate(1), deleter);
    alloc.construct(local_object, value);
    object = local_object.release();

Another way would be with try/catch:

    AllocatorType<object_t> alloc;
    object = alloc.allocate(1);
    try {
        alloc.construct(object, value);
    } catch(...) {
        alloc.deallocate(object, 1);
        throw;
    }

Beyond that, it would probably require wrapping the m_value and m_type in a struct, and moving the basic_json destructor to the new struct.

[ColinH]

Yes, either should work, not quite sure which style the author prefers.