103 lines
1.9 KiB
C++
103 lines
1.9 KiB
C++
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#pragma once
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#include <memory>
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#include <list>
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#include <functional>
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#include "sync.hh"
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#include "ref.hh"
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namespace nix {
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/* This template class implements a simple pool manager of resources
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of some type R, such as database connections. It is used as
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follows:
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class Connection { ... };
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Pool<Connection> pool;
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{
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auto conn(pool.get());
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conn->exec("select ...");
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}
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Here, the Connection object referenced by ‘conn’ is automatically
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returned to the pool when ‘conn’ goes out of scope.
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*/
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template <class R>
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class Pool
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{
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public:
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typedef std::function<ref<R>()> Factory;
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private:
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Factory factory;
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struct State
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{
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unsigned int count = 0;
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std::list<ref<R>> idle;
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};
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Sync<State> state;
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public:
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Pool(const Factory & factory = []() { return make_ref<R>(); })
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: factory(factory)
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{ }
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class Handle
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{
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private:
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Pool & pool;
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ref<R> r;
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friend Pool;
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Handle(Pool & pool, std::shared_ptr<R> r) : pool(pool), r(r) { }
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public:
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Handle(Handle && h) : pool(h.pool), r(h.r) { abort(); }
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Handle(const Handle & l) = delete;
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~Handle()
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{
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auto state_(pool.state.lock());
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state_->idle.push_back(r);
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}
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R * operator -> () { return &*r; }
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R & operator * () { return *r; }
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};
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Handle get()
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{
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{
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auto state_(state.lock());
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if (!state_->idle.empty()) {
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auto p = state_->idle.back();
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state_->idle.pop_back();
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return Handle(*this, p);
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}
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state_->count++;
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}
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/* Note: we don't hold the lock while creating a new instance,
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because creation might take a long time. */
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return Handle(*this, factory());
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}
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unsigned int count()
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{
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auto state_(state.lock());
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return state_->count;
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}
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};
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}
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