这是一步步的用EOSIO开发区块链DApp的第二部分，这部分将主要是为EOSIO平台开发智能合约。
示例智能合约的目的是模拟选举。我创建了一个EOSIO用户来托管智能合约。创建了两个公民用户来投票给候选人。投票记录保存在EOSIO区块链中。在此示例中，所有操作都在命令模式下运行。让我们开始吧。
开发智能合约EOSIO执行以WebAssembly标准开发的智能合约。所以我用C++开发了选举智能合约。以下是election.cpp的完整源代码：
#include <eosiolib/eosio.hpp> using namespace eosio; class election : public contract { private: // create the multi index tables to store the data /// @abi table struct candidate { uint64_t _key; // primary key std::string _name; // candidate name uint32_t _count = 0; // voted count uint64_t primary_key() const { return _key; } }; typedef eosio::multi_index<N(candidate), candidate> candidates; /// @abi table struct voter { uint64_t _key; uint64_t _candidate_key; // name of poll account_name _account; // this account has voted, avoid duplicate voter uint64_t primary_key() const { return _key; } uint64_t candidate_key() const { return _candidate_key; } }; typedef eosio::multi_index<N(voter), voter, indexed_by<N(_candidate_key), const_mem_fun<voter, uint64_t, &voter::candidate_key>>> voters; // local instances of the multi indexes candidates _candidates; voters _voters; uint64_t _candidates_count; public: election(account_name s) : contract(s), _candidates(s, s), _voters(s, s), _candidates_count(0) {} // public methods exposed via the ABI // on candidates /// @abi action void version() { print("Election Smart Contract version 0.0.1\n"); }; /// @abi action void addc(std::string name) { print("Adding candidate ", name, "\n"); uint64_t key = _candidates.available_primary_key(); // update the table to include a new candidate _candidates.emplace(get_self(), [&](auto &p) { p._key = key; p._name = name; p._count = 0; }); print("Candidate added successfully. candidate_key = ", key, "\n"); }; /// @abi action void reset() { // Get all keys of _candidates std::vector<uint64_t> keysForDeletion; for (auto &itr : _candidates) { keysForDeletion.push_back(itr.primary_key()); } // now delete each item for that poll for (uint64_t key : keysForDeletion) { auto itr = _candidates.find(key); if (itr != _candidates.end()) { _candidates.erase(itr); } } // Get all keys of _voters keysForDeletion.empty(); for (auto &itr : _voters) { keysForDeletion.push_back(itr.primary_key()); } // now delete each item for that poll for (uint64_t key : keysForDeletion) { auto itr = _voters.find(key); if (itr != _voters.end()) { _voters.erase(itr); } } print("candidates and voters reset successfully.\n"); }; /// @abi action void results() { print("Start listing voted results\n"); for (auto& item : _candidates) { print("Candidate ", item._name, " has voted count: ", item._count, "\n"); } }; /// @abi action void vote(account_name s, uint64_t candidate_key) { require_auth(s); bool found = false; // Did the voter vote before? for (auto& item : _voters) { if (item._account == s) { found = true; break; } } eosio_assert(!found, "You're voted already!"); // Findout the candidate by id std::vector<uint64_t> keysForModify; for (auto& item : _candidates) { if (item.primary_key() == candidate_key) { keysForModify.push_back(item.primary_key()); break; } } if (keysForModify.size() == 0) { eosio_assert(found, "Invalid candidate id!"); return; } // Update the voted count inside the candidate for (uint64_t key : keysForModify) { auto itr = _candidates.find(key); auto candidate = _candidates.get(key); if (itr != _candidates.end()) { _candidates.modify(itr, get_self(), [&](auto& p) { p._count++; }); print("Voted candidate: ", candidate._name, " successfully\n"); } } // Add this user to voters array _voters.emplace(get_self(), [&](auto& p) { p._key = _voters.available_primary_key(); p._candidate_key = candidate_key; p._account = s; }); }; }; EOSIO_ABI(election, (version)(reset)(addc)(results)(vote))注意最后一行EOSIO_ABI()是一个宏语句，用于自动生成ABI文件而不是手动编写。ABI文件用于定义提交动作处理程序。这告诉了EOSIO智能合约中处理程序的定义。
EOSIO为我们提供了多索引数据库API，可以将数据保存到区块链中。在上面的选举智能合约中，我定义了两个multi_index（类似于SQL表）：候选人和选民。实际上是两个数组存储两个结构：候选者和选民。我使用C++ STL来操作multi_index，例如add，update，delete。
请注意，两个结构在开头标有/// @abi table。这是告诉EOSIO abi生成器在election.abi文件中生成ABI表。这很方便。
编译选举智能合约：
$ eosiocpp -o election.wast election.cpp分别生成WAST和WASM文件。但这对EOSIO来说还不够。我们还需要生成ABI文件：
$ eosiocpp -g election.abi election.cpp Visual Studio Code的可选文件