在 Bitcoin 代码中，使用 Coin 类来表示单个交易对象中某个输出的币:

class Coin
 {
 public:
 //! unspent transaction output
 CTxOut out;
 //! whether containing transaction was a coinbase
 unsigned int fCoinBase : 1;
 //! at which height this containing transaction was included in the active block chain
 uint32_t nHeight : 31;
 .....
 .....
 }

数据元素除了CTxOut中的币值(nValue)、花费条件(scriptPubKey)之外, 还附带了一些元信息：是否是coinbase, 所在交易在哪个高度被打包进入 Blockchain。

再使用CCoinsView抽象类表达整个 blockchain 上的币的集合:

class CCoinsView
 {
 public:
 /** Retrieve the Coin (unspent transaction output) for a given outpoint.
 * Returns true only when an unspent coin was found, which is returned in coin.
 * When false is returned, coin's value is unspecified.
 */
 virtual bool GetCoin(const COutPoint &outpoint, Coin &coin) const; //! Just check whether a given outpoint is unspent.
 virtual bool HaveCoin(const COutPoint &outpoint) const; //! Retrieve the block hash whose state this CCoinsView currently represents
 virtual uint256 GetBestBlock() const; //! Retrieve the range of blocks that may have been only partially written.
 //! If the database is in a consistent state, the result is the empty vector.
 //! Otherwise, a two-element vector is returned consisting of the new and
 //! the old block hash, in that order.
 virtual std::vector GetHeadBlocks() const; //! Do a bulk modification (multiple Coin changes + BestBlock change).
 //! The passed mapCoins can be modified.
 virtual bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock); //! Get a cursor to iterate over the whole state
 virtual CCoinsViewCursor *Cursor() const; //! As we use CCoinsViews polymorphically, have a virtual destructor
 virtual ~CCoinsView() {} //! Estimate database size (0 if not implemented)
 virtual size_t EstimateSize() const { return 0; }
 };

Cinsview 类作为接口类，有很多具体实现子类:

CoinsViewDB类主要服务于从 Bitcoin 数据目录下的 chainstate 子目录下保存和读取存盘的 UTXO 集合:

class CCoinsViewDB final : public CCoinsView
 {
 protected:
 CDBWrapper db;
 public:
 explicit CCoinsViewDB(size_t nCacheSize, bool fMemory = false, bool fWipe = false);
 bool GetCoin(const COutPoint &outpoint, Coin &coin) const override;
 bool HaveCoin(const COutPoint &outpoint) const override;
 uint256 GetBestBlock() const override;
 std::vector GetHeadBlocks() const override;
 bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) override;
 CCoinsViewCursor *Cursor() const override;
 //! Attempt to update from an older database format. Returns whether an error occurred.
 bool Upgrade();
 size_t EstimateSize() const override;
 };

此类只有一个全局实例，在validation.cpp中定义:

std::unique_ptr pcoinsdbview;

在init.cpp中进程启动时， 会对改对象进行初始化:

pcoinsdbview.reset(new CCoinsViewDB(nCoinDBCache, false, fReset || fReindexChainState));

类CCoinsViewBacked本身没什么实际用处， 主要是作为多个Coinview层级之间的转接层， 它的数据成员 CCoinView *base 指向的就是后端即parent view , 如果某个coinsviewBacked的子类没有覆盖接口类CCoinsView 中的方法， 就会调用base指向的后端相应的方法。

class CCoinsViewBacked : public CCoinsView
 {
 protected:
 CCoinsView *base;public:
 CCoinsViewBacked(CCoinsView *viewIn);
 bool GetCoin(const COutPoint &outpoint, Coin &coin) const override;
 bool HaveCoin(const COutPoint &outpoint) const override;
 uint256 GetBestBlock() const override;
 std::vector GetHeadBlocks() const override;
 void SetBackend(CCoinsView &viewIn);
 bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) override;
 CCoinsViewCursor *Cursor() const override;
 size_t EstimateSize() const override;
 };

CCoinsViewBacked::CCoinsViewBacked(CCoinsView *viewIn) : base(viewIn) { }
 bool CCoinsViewBacked::GetCoin(const COutPoint &outpoint, Coin &coin) const { return base->GetCoin(outpoint, coin); }
 bool CCoinsViewBacked::HaveCoin(const COutPoint &outpoint) const { return base->HaveCoin(outpoint); }
 uint256 CCoinsViewBacked::GetBestBlock() const { return base->GetBestBlock(); }
 std::vector CCoinsViewBacked::GetHeadBlocks() const { return base->GetHeadBlocks(); }
 void CCoinsViewBacked::SetBackend(CCoinsView &viewIn) { base = &viewIn; }
 bool CCoinsViewBacked::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) { return base->BatchWrite(mapCoins, hashBlock); }
 CCoinsViewCursor *CCoinsViewBacked::Cursor() const { return base->Cursor(); }
 size_t CCoinsViewBacked::EstimateSize() const { return base->EstimateSize(); }

CCoinsViewErrorCatcher , CCoinsViewMemPool , CCoinsViewCache 三个定制实现在初始化时需要指定parent view,所以要继承于CCoinsViewBacked类。

coinsviewErrorCatcher 主要用途是包装对数据库读取做错误处理，后端是全局的磁盘实现pcoinsdbview。

class CCoinsViewErrorCatcher final : public CCoinsViewBacked
 {
 public:
 explicit CCoinsViewErrorCatcher(CCoinsView* view) : CCoinsViewBacked(view) {}
 bool GetCoin(const COutPoint &outpoint, Coin &coin) const override {
 try {
 return CCoinsViewBacked::GetCoin(outpoint, coin);
 } catch(const std::runtime_error& e) {
 uiInterface.ThreadSafeMessageBox(_("Error reading from database, shutting down."), "", CClientUIInterface::MSG_ERROR);
 LogPrintf("Error reading from database: %s\n", e.what());
 abort();
 }
 }
 };

启动时的初始化代码:

//init.cpp
 AppInitMain()
 pcoinscatcher.reset(new CCoinsViewErrorCatcher(pcoinsdbview.get()));

CCoinsViewCache 类是一个内存缓存的实现，内部使用hashmap 存储了某个outpoint 到Coin对象的映射，有一个全局实例pcoinsTip , 指向atctiveChain 的utxo，后端是磁盘实现CCoinsViewDB对象pcoinsdbview。

class CCoinsViewCache : public CCoinsViewBacked
 {
 protected:
 /**
 * Make mutable so that we can "fill the cache" even from Get-methods
 * declared as "const".
 */
 mutable uint256 hashBlock;
 mutable CCoinsMap cacheCoins; /* Cached dynamic memory usage for the inner Coin objects. */
 mutable size_t cachedCoinsUsage;
 ...
 ...
 }

启动时的初始化代码:

//init.cpp
 AppInitMain()
 pcoinsTip.reset(new CCoinsViewCache(pcoinscatcher.get()));

它的内部hashmap使用了定制的hash 方法siphash, 没有使用默认的std::hash方法(不是加密学安全的hash)， 估计是防止hash的key冲突,:

typedef std::unordered_map<COutPoint, CCoinsCacheEntry, SaltedOutpointHasher> CCoinsMap;class SaltedOutpointHasher
 {
 private:
 /** Salt */
 const uint64_t k0, k1;public:
 SaltedOutpointHasher();

size_t operator()(const COutPoint& id) const {
 return SipHashUint256Extra(k0, k1, id.hash, id.n);
 }
 };

这篇文章介绍了表示UTXO的相关表示的数据结构，下一篇文章将会UTXO的标记以及保存。