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This article will explain in detail how to use EOS multi-index table, the content of the article is of high quality, so the editor will share it with you for reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

In order to have an in-depth and clear understanding of the multi-index table, the final .cpp file section will be discussed in further detail. Note that the complete .cpp file can be found at the bottom of the page.

Glossary

Code: refers to the account_name that has announced a smart contract.

The range of data involved in scope:account_name.

Table_name: the name of the table stored in memory.

The structure to be stored by code decomposition

The data to be stored in a multi-index table is a limit_order structure. The primary_key (), get_expiration (), get_price () functions are used to return the table. The returned table is sorted according to the function called.

Struct limit_order {uint64_t id; uint128_t price; uint64_t expiration; account_name owner; auto primary_key () const {return id;} uint64_t get_expiration () const {return expiration;} uint128_t get_price () const {return price;} EOSLIB_SERIALIZE (limit_order, (id) (price) (expiration) (owner))}; create a multi-index table auto payer = ilm.get_account ();

Payer is a variable that holds the account. It adds billing elements to the multi-index table and modifies elements that are already in the multi-index table.

... eosio::multi_index

< N( orders ), limit_order, ... N(orders)是多索引表的名称，limit_order是要存储在表中的数据。 ... indexed_by< N( byexp ), const_mem_fun< limit_order, uint64_t, &limit_order::get_expiration>

>,.

Indexed_by

< N( byexp ), const_mem_fun< limit_order, uint64_t, &limit_order::get_expiration>

> defines the indexing method of multi-index tables. N (byexp) is the name of this index. Const_mem_fun indicates the data type being queried, the variable type of limit_order is uint64_t, and you will use the get_expiration function to get the variable.

... Indexed_by

< N( byprice ), const_mem_fun< limit_order, uint128_t, &limit_order::get_price>

>.

Indexed_by

< N( byprice ), const_mem_fun< limit_order, uint128_t, &limit_order::get_price>

> defines the indexing method of multi-index tables. N (byprice) is the name of this index. Const_mem_fun indicates the data type being queried, the variable type of limit_order is uint128_t, and you will use the get_price function to get the variable.

Orders (N (limitorders), N (limitorders))

Orders is a multi-index table.

Auto payer = ilm.get_account (); print ("Creating multi index table 'orders'.\ n"); eosio::multi_index

< N( orders ), limit_order, indexed_by< N( byexp ), const_mem_fun< limit_order, uint64_t, &limit_order::get_expiration>

>, indexed_by

< N( byprice ), const_mem_fun< limit_order, uint128_t, &limit_order::get_price>

> > orders (N (limitorders), N (limitorders)); add multiple index tables

Next, add two limit_order to the orders table. Note that payer is the "bill" account that is modifying the orders table.

Orders.emplace (payer, [&] (auto& o) {o.id = 1; o.expiration = 300; o.owner = N (dan);}); auto order2 = orders.emplace (payer, [&] (auto& o) {o.id = 2; o.expiration = 200; o.owner = N (thomas);}); sort by primary key

The default orders table is sorted by primary key.

Print ("Items sorted by primary key:\ n"); for (const auto& item: orders) {print ("ID=", item.id, ", expiration=", item.expiration, ", owner=", name {item.owner}, "\ n");} sort by second index expiration

The orders table is sorted by expiration and assigned to expidx.

Auto expidx = orders.get_index (); print ("Items sorted by expiration:\ n"); for (const auto& item: expidx) {print ("ID=", item.id, ", expiration=", item.expiration, ", owner=", name {item.owner}, "\ n");} sort by second index price

The orders table is sorted by price and assigned to oridx.

Auto pridx = orders.get_index (); print ("Items sorted by price:\ n"); for (const auto& item: pridx) {print ("ID=", item.id, ", expiration=", item.expiration, ", owner=", name {item.owner}, "\ n");} modify an input value

Next, the entry for "ID=2" is modified. Note that payer is the "bill" account that is modifying the orders table.

Print ("Modifying expiration of order with ID=2 to 400.\ n"); orders.modify (order2, payer, [&] (auto& o) {o.expiration = 400;}); get a minimum value auto lower = expidx.lower_bound; print ("First order with an expiration of at least 100 has ID=", lower- > id, "and expiration=", lower- > get_expiration (), "\ n"); complete .cpp file # include # include # include using namespace eosio Namespace limit_order_table {struct limit_order {uint64_t id; uint128_t price; uint64_t expiration; account_name owner; auto primary_key () const {return id;} uint64_t get_expiration () const {return expiration;} uint128_t get_price () const {return price } EOSLIB_SERIALIZE (limit_order, (id) (price) (expiration) (owner))}; class limit_order_table {public: ACTION (N (limitorders), issue_limit_order) {EOSLIB_SERIALIZE (issue_limit_order)}; static void on (const issue_limit_order& ilm) {auto payer = ilm.get_account () Print ("Creating multi index table 'orders'.\ n"); eosio::multi_index

< N( orders ), limit_order, indexed_by< N( byexp ), const_mem_fun< limit_order, uint64_t, &limit_order::get_expiration>

>, indexed_by

< N( byprice ), const_mem_fun< limit_order, uint128_t, &limit_order::get_price>

> orders (N (limitorders), N (limitorders)); orders.emplace (payer, [&] (auto& o) {o.id = 1; o.expiration = 300; o.owner = N (dan);}) Auto order2 = orders.emplace (payer, [&] (auto& o) {o.id = 2; o.expiration = 200; o.owner = N (thomas);}); print ("Items sorted by primary key:\ n") For (const auto& item: orders) {print ("ID=", item.id, ", expiration=", item.expiration, ", owner=", name {item.owner}, "\ n");} auto expidx = orders.get_index (); print ("Items sorted by expiration:\ n") For (const auto& item: expidx) {print ("ID=", item.id, ", expiration=", item.expiration, ", owner=", name {item.owner}, "\ n");} auto pridx = orders.get_index (); print ("Items sorted by price:\ n") For (const auto& item: pridx) {print ("ID=", item.id, ", expiration=", item.expiration, ", owner=", name {item.owner}, "\ n");} print ("Modifying expiration of order with ID=2 to 400.\ n") Orders.modify (order2, payer, [&] (auto& o) {o.expiration = 400;}); auto lower = expidx.lower_bound; print ("First order with an expiration of at least 100 has ID=", lower- > id, "and expiration=", lower- > get_expiration (), "\ n");} } / limit_order_tablenamespace limit_order_table {extern "C" {/ The apply method implements the dispatch of events to this contract void apply (uint64_t code, uint64_t action) {require_auth (code); eosio_assert (eosio::dispatch

< limit_order_table, limit_order_table::issue_limit_order >

(code, action), "Could not dispatch");} delete the table

The table cannot be deleted directly, but after all rows are deleted, the table is automatically deleted.

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