eos源码赏析（十四）：EOS智能合约数据持久化存储（下）

      template<typename Lambda>
      const_iterator emplace( uint64_t payer, Lambda&& constructor ) {
         using namespace _multi_index_detail;
         ......
         //1、做判断，不能在别的合约数据表内创建对象
         //2、创建对象并根据对象大小使用malloc分配内存
         //3、数据使用datastream写入对象
         ......
            auto pk = obj.primary_key();

            i.__primary_itr = db_store_i64( _scope, TableName, payer, pk, buffer, size );

            if ( max_stack_buffer_size < size ) {
               free(buffer);
            }

            if( pk >= _next_primary_key )
               _next_primary_key = (pk >= no_available_primary_key) ? no_available_primary_key : (pk + 1);
         //4、二级索引的处理

         const item* ptr = itm.get();
         auto pk   = itm->primary_key();
         auto pitr = itm->__primary_itr;

         _items_vector.emplace_back( std::move(itm), pk, pitr );

         return {this, ptr};
      }

*  Adds a new object (i.e., row) to the table.(在数据表中创建一个新的对象)
*  @brief Adds a new object (i.e., row) to the table.
*  @param payer - Account name of the payer for the Storage usage of the new object
//谁创建，谁支付内存消耗的费用
*  @param constructor - Lambda function that does an in-place initialization of the object to be created in the table
//Lambda函数，可以直接初始化刚才创建对的对象
*  @pre A multi index table has been instantiated //前置条件:multi-index已经被初始化
*  @post A new object is created in the Multi-Index table, with a unique primary key (as specified in the object).  The object is serialized and written to the table. If the table does not exist, it is created.
//使用emplace之后：
//带有唯一主键的新对象在multi-index表中被创建； 
//这个对象会被序列化，然后写入表中； 
//如果表不存在，则创建表。 
*  @post Secondary indices are updated to refer to the newly added object. If the secondary index tables do not exist, they are created.
//二级索引被更新，用以引用新添加的对象； 
//如果二级索引表不存在，则创建它们。 
*  @post The payer is charged for the storage usage of the new object and, if the table (and secondary index tables) must be created, for the overhead of the table creation.
//payer为创建新对象所使用的存储付费； 
//如果multi-index表和二级索引表需要被创建，则payer为表的创建付费。
 *  @return A primary key iterator to the newly created object
//返回一个新创建的对象的主键迭代器
*  Exception - The account is not authorized to write to the table.
//异常：没有权限去写入这张数据表

//get
const T& get( uint64_t primary, const char* error_msg = "unable to find key" )const {
   auto result = find( primary );
   eosio_assert( result != cend(), error_msg );
   return *result;
   }
//find
const_iterator find( uint64_t primary )const {
   auto itr2 = std::find_if(_items_vector.rbegin(), _items_vector.rend(), [&](const item_ptr& ptr) {
   return ptr._item->primary_key() == primary;
   });
   if( itr2 != _items_vector.rend() )
       return iterator_to(*(itr2->_item));

   auto itr = db_find_i64( _code, _scope, TableName, primary );
       if( itr < 0 ) return end();

       const item& i = load_object_by_primary_iterator( itr );
       return iterator_to(static_cast<const T&>(i));
   }

void tianlongbabu::create(account_name heroname,uint64_t heroforceidx,uint64_t heroinsideidx,string herodes,string heroborn)
{
    print(name{heroname},"出生在:",heroborn,",现在是",herodes,",他的武力值:",heroforceidx,",他的内力值:",heroinsideidx);
    uint64_t heroindex = 0;
    auto dbhero = ht.emplace(_this_contract,[&](auto &p)
    {
        p.heroid = ht.available_primary_key();   //主键自增
        heroindex = p.heroid;
        p.herodes        = herodes;
        p.heroborn       = heroborn;
        p.heroforceidx   = heroforceidx;
        p.heroinsideidx  = heroinsideidx;

    });
    print("--emplace返回该英雄的编号:",dbhero->heroid);
}
//push action
ubuntu@VM-16-6-ubuntu:~/eos/contracts/tlbb$ cleos push action tlbb.code create '["xiaofeng","1000","500","丐帮帮主","辽国"]' -p xiaofeng
executed transaction: a92821dacc552d8cb9a9b60330a878b4d95e32a11a148fe7559a7189d6253e89  136 bytes  1274 us
#     tlbb.code <= tlbb.code::create            {"heroname":"xiaofeng","heroforceidx":1000,"heroinsideidx":500,"herodes":"丐帮帮主","heroborn":"...
>> xiaofeng出生在:辽国,现在是丐帮帮主,他的武力值:1000,他的内力值:500--emplace返回该英雄的编号:0

//关于hero_table的multi-index的声明修改如下：
typedef eosio::multi_index<N(heros),heros,indexed_by<N(heroforceidx),const_mem_fun<heros,uint64_t,&heros::get_heroforceidx>>> heros_table;
heros_table ht;
//根据武力值二级索引来查找天龙英雄的相关信息
void tianlongbabu::selectbyfidx(uint64_t heroforceidx)
{
    auto force_index = ht.get_index<N(heroforceidx)>();
    auto findhero = force_index.lower_bound(heroforceidx);
    eosio_assert(findhero != force_index.end(),"您查找的英雄不存在");
    print("武力值为",heroforceidx,"的英雄其编号为",findhero->heroid);
}
//push action
ubuntu@VM-16-6-ubuntu:~/eos/contracts/tlbb$ cleos push action tlbb.code selectbyfidx '["1000"]' -p tlbb.code
executed transaction: b2ccddeacabe7f9b5abe7f92725dd91c0f028ed1da414f276597d821def3c9dd  104 bytes  1013 us
#     tlbb.code <= tlbb.code::selectbyfidx      {"heroforceidx":1000}
>> 武力值为1000的英雄其编号为0

int apply_context::db_store_i64( uint64_t code, uint64_t scope, uint64_t table, const account_name& payer, uint64_t id, const char* buffer, size_t buffer_size ) {
//   require_write_lock( scope );
   const auto& tab = find_or_create_table( code, scope, table, payer );
   auto tableid = tab.id;

   FC_ASSERT( payer != account_name(), "must specify a valid account to pay for new record" );

   const auto& obj = db.create<key_value_object>( [&]( auto& o ) {
      o.t_id        = tableid;
      o.primary_key = id;
      o.value.resize( buffer_size );
      o.payer       = payer;
      memcpy( o.value.data(), buffer, buffer_size );
   });

   db.modify( tab, [&]( auto& t ) {
     ++t.count;
   });
         //而后我们再来看db.create，其最终还是会走向chainbase中的emplace
         /**
          * Construct a new element in the multi_index_container.//使用boost::multi-index容器创建一个新的元素
          * Set the ID to the next available ID, then increment _next_id and fire off on_create().//将id设置为下一个可用id
          */
         template<typename Constructor>
         const value_type& emplace( Constructor&& c ) {
            auto new_id = _next_id;

            auto constructor = [&]( value_type& v ) {
               v.id = new_id;
               c( v );
            };

            auto insert_result = _indices.emplace( constructor, _indices.get_allocator() );

            if( !insert_result.second ) {
               BOOST_THROW_EXCEPTION( std::logic_error("could not insert object, most likely a uniqueness constraint was violated") );
            }

            ++_next_id;
            on_create( *insert_result.first );
            return *insert_result.first;
         }