下面演示EOS智能合约编写，实例将涉及对链上多索引（multi_index）的增删改查。

实例为License合约，主要功能为

• 客户设置自己的信息
• 客户创建自己的项目
• 客户购买项目授权
• 验证项目授权
  （由于演示二级索引，项目名称为索引唯一，所以实例为用户单项目）
  上面的权限都是基于客户自身的权限验证，创建是创建的系统子账户，可用于系统币转账

合约代码基于C++语言，源代码分为源文件（.cpp）和头文件(.hpp)
//License.hpp

#include <eosiolib/asset.hpp>
#include <eosiolib/eosio.hpp>
#include <eosiolib/print.hpp>

class License : public eosio::contract {
    public:
        License( account_name self )
        :eosio::contract(self)
        ,accounts(_self, _self)
        ,projecttable(_self, _self){}

        void setuserinfo(account_name owner, const std::string& company_name, const std::string& contact_info);
        void makeproject(account_name owner, const std::string& project_name);
        void buylicense(account_name owner, const std::string& project_name);
        void testlicense(account_name owner, const std::string& project_name);

    private:
        //@abi table accounts i64
        struct account{
            account_name name;
            std::string company_name;
            std::string contact_info;
            uint8_t level;
            uint64_t primary_key()const { return name; }
            EOSLIB_SERIALIZE(account, (name)(company_name)(contact_info)(level))
        };

        typedef eosio::multi_index<N(accounts),account> account_table;
        account_table accounts;

        //@abi table projecttable i64
        struct project{
            uint32_t pkey;
            account_name owner_name;
            std::string project_name;
            std::string company_name;
            std::string contact_info;
            uint8_t      status;

            uint64_t primary_key()const { return pkey; }
            uint64_t getowner_name() const{return owner_name;}
            EOSLIB_SERIALIZE(project, (pkey)(owner_name)(project_name)(company_name)(contact_info)(status))
        };
        //contracts/eosiolib/multi_index.hpp
        //multi_index用于封装智能合约对数据库的管理，增、删、改、查
        //eosio::multi_index<N(表名),数据类型> 
        //一个multi_index最多支持16个二级索引
        //定义二级索引使用eosio::indexed_by模板。N(byowner_name)为索引的名称
        //project表数据类型，uint64_t索引类型即索引函数返回的类型(只支持uint64_t,uint128_t,double,long double,key256类型)，&project::getowner_name索引函数。
        typedef eosio::multi_index<N(projecttable),project, eosio::indexed_by<N(byowner_name), eosio::const_mem_fun<project, account_name, &project::getowner_name> >> projecttable_type;
        projecttable_type projecttable;
};
EOSIO_ABI(License, (setuserinfo)(makeproject)(buylicense)(testlicense))

//License.cpp

#include <License.hpp>

void License::setuserinfo(account_name owner, const std::string& company_name, const std::string& contact_info){
    eosio::print("License::setuserinfo line 4");
    require_auth(owner);
    auto account_itr = accounts.find(owner);
    if(account_itr == accounts.end()){
        account_itr = accounts.emplace(_self, [&](auto& account){
            account.name = owner;
            account.company_name = company_name;
            account.contact_info = contact_info;
        });
    }
    else{
        accounts.modify(account_itr, 0, [&](auto& account) {
            account.company_name = company_name;
            account.contact_info = contact_info;
        });
    }
}

void License::makeproject(account_name owner, const std::string& project_name){
    require_auth(owner);
    //二级索引查找
    auto customer_index = projecttable.template get_index<N(byowner_name)>();
    account_name customer_acct = owner; //如果是string 可以用 eosio::string_to_name 转换
    auto cust_itr = customer_index.find(customer_acct);
    if (cust_itr != customer_index.end() && cust_itr->owner_name == customer_acct) {
        customer_index.modify(cust_itr, 0, [&](auto& project) {
            project.project_name = project_name;
            project.status = cust_itr->status; //初始化未授权，须另购买
            auto account_itr = accounts.find(owner);
            if(account_itr != accounts.end()){
                project.project_name = project_name;
                project.company_name = account_itr->company_name;
                project.contact_info = account_itr->contact_info;

            }
        });
    } 
    else{
        auto project_itr = projecttable.emplace(_self, [&](auto& project){
                project.owner_name = owner;
                project.project_name = project_name;
                auto account_itr = accounts.find(owner);
                if(account_itr != accounts.end()){
                    project.company_name = account_itr->company_name;
                    project.contact_info = account_itr->contact_info;
                    project.status = 0; //初始化未授权，须另购买
                }
            });
    }
}

void License::buylicense(account_name owner, const std::string& project_name){
    require_auth(owner);
    bool bFind = false;
    //二级索引查找
    auto customer_index = projecttable.template get_index<N(byowner_name)>();
    account_name customer_acct = owner; //如果是string 可以用 eosio::string_to_name 转换
    auto cust_itr = customer_index.find(customer_acct);
    while (cust_itr != customer_index.end() && cust_itr->owner_name == customer_acct) {
        if(cust_itr->project_name == project_name){
            if(cust_itr->status == 0){
                //TODO 判断当前账户余额，购买转账等操作
                customer_index.modify(cust_itr, 0, [&](auto& project) {
                    project.status = 1;
                });
            }
            else{
                eosio_assert(false, "The current project is authorized" );
            }
            bFind = true;
            break;
        }
        cust_itr++;
    } 
    eosio_assert(bFind, "owner with project not exists" );
}

void License::testlicense(account_name owner, const std::string& project_name){
    require_auth(owner);
    bool bFind = false;
    for( const auto& project : projecttable ) {
        if(project.project_name == project_name && project.owner_name == owner){
            eosio_assert((project.status == 1), "The current project is not authorized" );
            bFind = true;
            break;
        }
    }
    eosio_assert(bFind, "owner with project not exists" );
}

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