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