主要分析内容：
bvar::Adder<int> test;
test << 1;
test.get_value();

1、operator<<

Adder定义：

template <typename T>
class Adder : public Reducer<T, detail::AddTo<T>, detail::MinusFrom<T> > {
public:
    typedef Reducer<T, detail::AddTo<T>, detail::MinusFrom<T> > Base;
    typedef T value_type;
    typedef typename Base::sampler_type sampler_type;
public:
    Adder() : Base() {}
    explicit Adder(const butil::StringPiece& name) : Base() {
        this->expose(name);
    }
    Adder(const butil::StringPiece& prefix,
          const butil::StringPiece& name) : Base() {
        this->expose_as(prefix, name);
    }
    ~Adder() { Variable::hide(); }
};

operator<<：
定义在Reducer中。

template <typename T, typename Op, typename InvOp>
inline Reducer<T, Op, InvOp>& Reducer<T, Op, InvOp>::operator<<(
    typename butil::add_cr_non_integral<T>::type value) {
    // It's wait-free for most time
    agent_type* agent = _combiner.get_or_create_tls_agent();
    if (__builtin_expect(!agent, 0)) {
        LOG(FATAL) << "Fail to create agent";
        return *this;
    }
    agent->element.modify(_combiner.op(), value);
    return *this;
}

Reducer中

typedef typename detail::AgentCombiner<T, T, Op> combiner_type;
...
combiner_type   _combiner;
...

AgentCombiner构造函数：
AgentGroup::create_new_agent()保证了id的唯一。

    explicit AgentCombiner(const ResultTp result_identity = ResultTp(),
                           const ElementTp element_identity = ElementTp(),
                           const BinaryOp& op = BinaryOp())
        : _id(AgentGroup::create_new_agent())
        , _op(op)
        , _global_result(result_identity)
        , _result_identity(result_identity)
        , _element_identity(element_identity) {
    }

_combiner.get_or_create_tls_agent()

    // We need this function to be as fast as possible.
    inline Agent* get_or_create_tls_agent() {
        Agent* agent = AgentGroup::get_tls_agent(_id);
        if (!agent) {
            // Create the agent
            agent = AgentGroup::get_or_create_tls_agent(_id);
            if (NULL == agent) {
                LOG(FATAL) << "Fail to create agent";
                return NULL;
            }
        }
        if (agent->combiner) {
            return agent;
        }
        agent->reset(_element_identity, this);
        // TODO: Is uniqueness-checking necessary here?
        {
            butil::AutoLock guard(_lock);
            _agents.Append(agent);
        }
        return agent;
    }

2、get_value

    // Get reduced value.
    // Notice that this function walks through threads that ever add values
    // into this reducer. You should avoid calling it frequently.
    T get_value() const {
        CHECK(!(butil::is_same<InvOp, detail::VoidOp>::value) || _sampler == NULL)
            << "You should not call Reducer<" << butil::class_name_str<T>()
            << ", " << butil::class_name_str<Op>() << ">::get_value() when a"
            << " Window<> is used because the operator does not have inverse.";
        return _combiner.combine_agents();
    }

combine_agents:

    // [Threadsafe] May be called from anywhere
    ResultTp combine_agents() const {
        ElementTp tls_value;
        butil::AutoLock guard(_lock);
        ResultTp ret = _global_result;
        for (butil::LinkNode<Agent>* node = _agents.head();
             node != _agents.end(); node = node->next()) {
            node->value()->element.load(&tls_value);
            call_op_returning_void(_op, ret, tls_value);
        }
        return ret;
    }