Develop and API introduction

Naming Rules

In the process of generating the abi interface file, will base32 encode the ACTION name and TABLE name, so there are restrictions on the naming rules.

ACTION, TABLE range: [1 ~ 5 a ~ z]

ACTION, TABLE length: no more than 13

class name range: [a ~ z 0 ~ 9]

class name length: unlimited

Built-in type

The contract provides built-in types in addition to the basic types suppoerted by c++ syntax.

contract_asset

contract_asset class contains two fields

int64_t     amount;
uint64_t    asset_id;

Use the is_amount_within_range member method to verify that the contract_asset amount is valid or not

bool is_amount_within_range() const {
    return -max_amount <= amount && amount <= max_amount;
}

Note

In the contract_asset type, where amount represents the number of assets and it uses large members to store in the chain . For example, the accuracy of GXC assets is 5,1 GXC actually needs to be expressed as 1 * 100000 = 100000, that means, amount is 100000.

The asset_id indicates the instance_id of the asset (for example, if the asset id is 1.3.1, its instance_id is 1).

You can learn the use of the contract_asset type refer to the Bank contract.

signature

the definition of signature class

struct signature {
   uint8_t data[65];
};

example：

void verify(checksum256 hash, std::string pub_key, signature sig)
{   
    print(pub_key, "\n");
    assert_recover_key(&hash, &sig, pub_key.c_str(), pub_key.length());
}

public_key

public_key class

struct public_key {
   char data[33];
};

checksum256

checksum256 class

struct ALIGNED(checksum256) {
   uint8_t hash[32];
};

example：

void issue(const std::string& question, const checksum256& hashed_answer)
{   
    uint64_t owner = get_trx_sender();
    records.emplace(owner, [&](auto &p) {
            p.issuer = owner;
            p.question = question;
            p.hashed_answer = hashed_answer;
    });
}  

checksum160

checksum160 class

struct ALIGNED(checksum160) {
   uint8_t hash[20];
};

checksum512

checksum512 class

struct ALIGNED(checksum512) {
   uint8_t hash[64];
};

block_id_type

block_id_type class

typedef struct checksum160      block_id_type;

Built-in API

When you develop smart contracts on GXChain, you can call the built-in API to implement richer features.

belong	api name	description
<graphenelib/action.h>	current_receiver	Returns the instance_id of the current contract account( the last digit of the account id)
<graphenelib/action.h>	get_action_asset_id	Returns the asset instance_id sent to the contract by this call( the last digit of the asset id)
<graphenelib/action.h>	get_action_asset_amount	Returns the number of assets sent to the contract by this call
<graphenelib/asset.h>	withdraw_asset	Transfer assets from the current contract account to the other account
<graphenelib/asset.h>	get_balance	Get the balance of an asset in the account on the chain
<graphenelib/crypto.h>	sha1	Obtain the encrypted data of the sha1 algorithm
<graphenelib/crypto.h>	sha256	Obtain the encrypted data of the sha256 algorithm
<graphenelib/crypto.h>	sha512	Obtain the encrypted data of the sha512 algorithm
<graphenelib/crypto.h>	ripemd160	Obtain the encrypted data of the ripemd160 algorithm
<graphenelib/crypto.h>	assert_recover_key	Verify that the given signature and hash can recover the public key
<graphenelib/global.h>	get_head_block_num	Get the head block number
<graphenelib/global.h>	get_head_block_id	Get the head block hash
<graphenelib/global.h>	get_block_id_for_num	Get the specified block hash
<graphenelib/global.h>	get_head_block_time	Get the time of the block, return the value in seconds
<graphenelib/global.h>	get_trx_sender	Get the instance_id of the account that called the contract
<graphenelib/global.h>	get_trx_origin	Get the instance_id of the original account
<graphenelib/global.h>	get_account_id	Get the instance_id of the account by the account name
<graphenelib/global.h>	get_account_name_by_id	Obtain an account name by the account instance id
<graphenelib/global.h>	get_asset_id	Get the instance_id of the asset by the asset name
<graphenelib/global.h>	read_transaction	Read serialized data of the current transaction
<graphenelib/global.h>	transaction_size	Get the length of the current transaction data after it has been serialized
<graphenelib/global.h>	expiration	Get expiration time of the transaction
<graphenelib/global.h>	tapos_block_num	Returns the block number referenced by the transaction
<graphenelib/global.h>	tapos_block_prefix	Return the block ID referenced by the transaction (the second 32-bit number)
<graphenelib/action.h>	read_action_data	Read data of the current action
<graphenelib/action.h>	action_data_size	Returns the number of bytes needed to get the current action data
<graphenelib/action.h>	send_inline	Inline execute action
<graphenelib/action.hpp>	unpack_action_data	Deserialize the current action data into a defined action object
<graphenelib/system.h>	graphene_assert	If the assertion fails, interrupt the execution of this contract and roll all states
<graphenelib/system.h>	graphene_assert_message	If the assertion fails, interrupt the execution of this contract and roll all states
<graphenelib/system.h>	print	Print log

current_receiver

Function: uint64_t current_receiver()

Head file: <graphenelib/action.h>

Description: Returns the instance_id of the current contract account( the last digit of the account id)

Return value: Instance_id

Example:

// @abi action
void examcurr(){
    uint64_t ins_id = current_receiver();
    print("current contract account id: ", ins_id, "\n");
}

get_action_asset_id

Function: uint64_t get_action_asset_id()

Head file: <graphenelib/action.h>

Description: Return the asset instance_id sent to the contract by this call

Return value: Returns 0 means the action has no attached assets, otherwise returns instance_id

Example:

// @abi action
void examgetast(){
    uint64_t ast_id = get_action_asset_id();
    print("call action asset id: ",ast_id, "\n");
}

get_action_asset_amount

Function: int64_t get_action_asset_amount()

Head file: <graphenelib/action.h>

Description: Returns the number of assets sent to the contract by this call

Return value: Returns 0 means the contract has no accompanying assets, otherwise means the number of assets attached and the quantity needs to be divided by 100,000

Example:

//get_action_asset_amount
// @abi action
void examgetamo(){
    int64_t amount = get_action_asset_amount();
    print("call action asset amount: ",amount,"\n");      
}

inline_transfer

Function:void inline_transfer(uint64_t from, uint64_t to, uint64_t asset_id, int64_t amount, const char* data, uint32_t length)

Head file:<graphenelib/asset.h>

Description: Transfer the assets of the current contract to an external account

Params:

Params	Type	Description
from	uint64_t	Must be _self
to	uint64_t	The instance_id of the receiver account
asset_id	uint64_t	Asset id used to transfer
amount	int64_t	Transfer amount, this number contains the accuracy of the asset, for example, if you want to transfer 1 GXC, you should write 100000
data	const char*	The first address of the data string(memo)
length	uint32_t	Length of the data string

Example:

// @abi action
void examwith(uint64_t from,uint64_t to, uint64_t asset_id, int64_t amount){
    std::string memo = "withdraw";
    inline_transfer(from,to,asset_id,amount, memo.c_str(), memo.size());
    print("inline_transfer example\n");
}

withdraw_asset

Function: void withdraw_asset(uint64_t from, uint64_t to, uint64_t asset_id, int64_t amount)

Head file: <graphenelib/asset.h>

Description: Transfer assets from the current contract account to the other account

Params:

Params	Type	Description
from	uint64_t	Must be _self
to	uint64_t	The instance_id of the receiver account
asset_id	uint64_t	Asset id used to transfer
amount	int64_t	Transfer amount, this number contains the accuracy of the asset, for example, if you want to transfer 1 GXC, you should write 100000

Example:

// @abi action
void examwith(uint64_t from,uint64_t to, uint64_t asset_id, int64_t amount){
    withdraw_asset(from,to,asset_id,amount);
    print("withdraw_asset example\n");
}

get_balance

Function: int64_t get_balance(int64_t account, int64_t asset_id)

Head file: <graphenelib/asset.h>

Description: Get the balance of an asset in the account on the chain

Return value: The balance

Params:

Params	Type	Description
account	int64_t	Account's instance_id
asset_id	int64_t	Asset's instance_id

Example:

// @abi action
void examgetbl(int64_t account, int64_t asset_id){
    int64_t balance = get_balance(account, asset_id);
    print("account balance: ",balance,"\n");
}

sha1

Function: void sha1(const char *data, uint32_t length, checksum160 *hash)

Head file: <graphenelib/crypto.h>

Description: Obtain the encrypted data of the sha1 algorithm

Params:

Params	Type	Description
data	const char*	Data start address
length	uint32_t	Data length
hash	checksum160*	Hash value

Example:

// @abi action
void examsha1(std::string data){
    checksum160 hash;
    sha1(data.c_str(),data.length(),&hash);
    printhex(hash.hash,20);
    print("\n");
}

sha256

Function: void sha256(const char *data, uint32_t length, checksum256 * hash)

Head file: <graphenelib/crypto.h>

Description: Obtain the encrypted data of the sha256 algorithm

Params:

Params	Type	Description
data	const char*	Data start address
length	uint32_t	Data length
hash	checksum256*	Hash value

Example:

// @abi action
void examsha25(std::string data){
    checksum256 hash;
    sha256(data.c_str(),data.length(),&hash);
    printhex(hash.hash,32);
    print("\n");
}

sha512

Function: void sha512(const char *data, uint32_t length, checksum512 * hash)

Head file: <graphenelib/crypto.h>

Description: Obtain the encrypted data of the sha512 algorithm

Params:

Params	Type	Description
data	const char*	Data start address
length	uint32_t	Data length
hash	checksum512*	Hash value

Example:

// @abi action
void examsha512(std::string data){
    checksum512 hash;
    sha512(data.c_str(),data.length(),&hash);
    printhex(hash.hash,64);
    print("\n");
}

ripemd160

Function: void ripemd160(const char *data, uint32_t length, checksum160 * hash)

Head file: <graphenelib/crypto.h>

Description: Obtain the encrypted data of the ripemd160 algorithm

Params:

Params	Type	Description
data	const char*	Data start address
length	uint32_t	Data length
hash	checksum160*	Hash value

Example:

// @abi action
void examripemd(std::string data){
    checksum160 hash;
    ripemd160(data.c_str(),data.length(),&hash);
    printhex(hash.hash,20);
    print("\n");
}

assert_recover_key

Function: void assert_recover_key(const checksum256 *digest,const signature *sig, const char *pub, uint32_t publen);

Head file: <graphenelib/crypto.h>

Description: Verify that the given signature and hash can recover the public key

Params:

Params	Type	Description
data	const checksum256*	Sha256 hash
sig	const signature*	Signature value of the string
pub	const char*	Public key(in hexadecimal format)
publen	uint32_t	Plulic key length

Example:

// @abi action
void examrecover(checksum256 dig,signature sig,std::string pkey)
{
    assert_recover_key(&dig,&sig,pkey.c_str(), pkey.length());
}

get_head_block_num

Function: int64_t get_head_block_num()

Head file: <graphenelib/global.h>

Description: Return the latest block number

Return value: The head block number

Example:

// @abi action
void examgetnum(){
    int64_t head_num = get_head_block_num();
    print("head block num: ",head_num, "\n");
}

get_head_block_id

Function: void get_head_block_id(checksum160* hash)

Head file: <graphenelib/global.h>

Description: Get the head block hash

Params:

Params	Type	Description
hash	checksum160*	The head block hash value

Example:

// @abi action
void examgetid(){
    checksum160 block_hash;
    get_head_block_id(&block_hash);
    printhex(block_hash.hash,20);
    print("\n");
}

get_block_id_for_num

Function: void get_block_id_for_num(checksum160* hash, uint32_t block_num)

Head file: <graphenelib/global.h>

Description: Get the specified block hash

Params:

Params	Type	Description
hash	checksum160*	The specified block hash
block_num	uint32_t	The specified block num

Example:

// @abi action
void examidnum(){
    checksum160 block_hash;
    get_block_id_for_num(&block_hash,1);             //get the hash of first block
    printhex(block_hash.hash,20);
    print("\n");
}

get_head_block_time

Function: int64_t get_head_block_time()

Head file: <graphenelib/global.h>

Description: Get the time of the latest block, return the value in seconds

Return Value: The time of the latest block

Example:

// @abi action
void examgettime(){
    int64_t head_time;
    head_time = get_head_block_time();
    print("head block time: ",head_time,"\n");
}

get_trx_sender

Function: uint64_t get_trx_sender()

Head file: <graphenelib/global.h>

Description: Get the instance_id of the account that called the contract

Return Value: Instance_id of the account

Example:

// @abi action
void examgettrx(){
    uint64_t sender_id;
    sender_id = get_trx_sender();
    print("call action instance id: ",sender_id, "\n");
}

get_trx_origin

Function: uint64_t get_trx_origin()

Head file: <graphenelib/global.h>

Description: Get the instance_id of the original account

Return Value: Original account instance_id

Example:

// @abi action
void examgetori(){
    uint64_t origin_id;
    origin_id = get_trx_origin();
    print("original instance id: ",origin_id, "\n");
}

get_account_id

Function: int64_t get_account_id(const char * data, uint32_t length)

Head file: <graphenelib/global.h>

Description: Get the instance_id of the account by the account name

Return Value: Returns the instance_id of the account if the account exists, or -1 if the account does not exist

Params:

Params	Type	Description
data	const char*	Account name( such as nathan)
length	uint32_t	Account name length( such as the length of nathan is 6)

Example:

// @abi action
void examgetacid(std::string data){
    int64_t acc_id;
    acc_id = get_account_id(data.c_str(), data.length());
    print("account id: ",acc_id, "\n");
}

get_account_name_by_id

Function: int64_t get_account_name_by_id(array_ptr<char> data, size_t buffer_size, int64_t account_id)

Head file: <graphenelib/global.h>

Description: Obtain an account name by the account instance id

Return Value: Returns 0 if the account exists, or -1 if the account does not exist

Params:

Params	Type	Description
data	const char*	Account name
length	uint32_t	Account name length
account_id	int64_t	Account instance_id

Example:

// @abi action
void examgetname(int64_t accid){
    char data[65]={0};
    int64_t result;
    result = get_account_name_by_id(data,65,accid);
    print(static_cast<const char*>data,"\n");
}

get_asset_id

Function: int64_t get_asset_id(const char * data, uint32_t length)

Head file: <graphenelib/global.h>

Description: Get the instance_id of the asset by the asset name

Return Value: Returns the instance_id of the asset if the asset exists, or -1 if the asset does not exist

Params:

Params	Type	Description
data	const char*	Asset name
length	uint32_t	The length of asset name

Example:

// @abi action
void examassid(std::string data){
    int64_t assid;
    assid = get_asset_id(data.c_str(),data.length());
    print("asset id: ",assid, "\n");
}

read_transaction

Function: int read_transaction(char* dst, uint32_t dst_size)

Head file: <graphenelib/global.h>

Description: Extract serialized data of the current transaction to the param dst

Return Value: When dst_size is 0, it returns the number of bytes required for reading; when dst_size is not 0, it returns the number of bytes actually read (The minimum value of dst_size is also the minimum value of transaction size )

Params:

Params	Type	Description
dst	char*	The pointer to receive the read buffer data
dst_size	uint32_t	The read data size

Example:

// @abi action
void examreadtrx(){
    int dwsize;
    dwsize =transaction_size();
    char* pBuffer = new char[dwsize];
    uint32_t size = read_transaction(pBuffer,dwsize);
    delete[] pBuffer;
}

transaction_size

Function: int transaction_size()

Head file: <graphenelib/global.h>

Description: Get the length of the data of the current transaction which has been serialized

Return Value: The length of the data

Example:

// @abi action
void examtrxsize(){
    int dwsize;
    dwsize =transaction_size();
    print("the size of the serialize trx: ",dwsize,"\n");
}

expiration

Function: uint64_t expiration()

Head file: <graphenelib/global.h>

Description: Get transaction expiration time

Return Value: The transaction expiration time

Example:

// @abi action
void exampira(){
    uint64_t timenum = expiration();
    print("the expiration time: ", timenum, "\n");
}

tapos_block_num

Function: int tapos_block_num()

Head file: <graphenelib/global.h>

Description: Returns the block number referenced by the transaction

Return Value: The block number

Example:

// @abi action
void examtapnum(){
    uint64_t tapos_num;
    tapos_num = tapos_block_num();
    print("ref block num: ",tapos_num, "\n");
}

tapos_block_prefix

Function: uint64_t tapos_block_prefix()

Head file: <graphenelib/global.h>

Description: Get the block ID of the transaction reference (the second 32-bit number)

Return Value: Return the block ID of the transaction reference (the second 32-bit number)

Example:

// @abi action
void examtappre(){
    uint64_t tapos_prefix;
    tapos_prefix = tapos_block_prefix();
    print("ref block id: ",tapos_prefix, "\n");
}

read_action_data

Function: uint32_t read_action_data( void* msg, uint32_t len )

Head file: <graphenelib/action.h>

Description: Read the data of the current action

Return Value: Return the number of bytes of data actually read. If len is 0, the number of bytes required for reading will be returned. Params:

Params	Type	Description
msg	void*	The pointer to receive the read buffer data
len	uint32_t	Buffer size

Example:

// @abi action
void examract(uint64_t num,std::string number){
    auto size = action_data_size();
    char *buffer = static_cast<char*>(malloc(size));
    read_action_data((void*)buffer,size);
    printhex(buffer,size);
}

action_data_size

Function: uint32_t action_data_size()

Head file: <graphenelib/action.h>

Description: Returns the number of bytes required to read the current action data

Return Value: The number of bytes

Example:

// @abi action
void examrasize(uint64_t num,std::string number){
    auto size = action_data_size();
    print("size: ", size, "\n");
}

send_inline

Function: void send_inline(char *serialized_action, size_t size)

Head file: <graphenelib/action.h>

Description: Inline execute action（ Generally, by constructing an action and inlining the action through its send member method, its internal implementation is send_inline，Inline action）

unpack_action_data

Function: T unpack_action_data()

Head file: <graphenelib/action.hpp>

Description: Deserialize the current action data into a defined action object

Return Value: Action object

Example:

struct myaction {
    uint64_t num;
    std::string name;

    GRAPHENE_SERIALIZE(myaction,(num)(name))
};
// @abi action
void examupact(uint64_t num,std::string name){
    auto my = unpack_action_data<myaction>();
    print(my.name, "\n");
}

graphene_assert

Function: void graphene_assert(uint32_t test, const char* msg)

Head file: <graphenelib/system.h>

Description: If the assertion fails, interrupt the execution of this contract and roll all states

Params:

Params	Type	Description
test	uint32_t	Assertion
msg	const char*	Message

Example:

// @abi action
void examassert(){
    uint64_t number=1;
    graphene_assert(number == 1, "wrong!");
}

graphene_assert_message

Function: void graphene_assert_message(uint32_t test, const char* msg, uint32_t msg_len)

Head file: <graphenelib/system.h>

Description: If the assertion fails, interrupt the execution of this contract and roll all states

Params:

Params	Type	Description
test	uint32_t	Assertion
msg	const char*	Message
msg_len	uint32_t	Message size

Example:

// @abi action
void examassmsg(){
    uint64_t number=1;
    std::string msg = "wrong!!!";
    graphene_assert_message(number == 1, msg.c_str(),msg.length());
}

print

Function: void print(const char* ptr)

Head file: <graphenelib/system.h>

Description: Print log（Please click Contract Debug）

Params:

Params	Type	Description
ptr	const char*	Log

Example:

// @abi action
void examprint(){
    print("example example example!!!\n");
}

example

The sample contract for api usage has been deployed to the test network, which can be tested by the IDE client, click to view contract source,contract name is apitest3

Inline action

Description

GXChain supports one contract to call another contract and supports payment accounts to set ram fees. For examples :User --> contract_A --> contract_B, for contract contract_B, User is the original caller, and contract_A is sender.

However, cross-contract calls are limited by the number of layers: no more than three contracts in call chain. Such as : User -> contract_A -> contract_B -> contract_C, if the number of call levels is greater than 3, the execution will be terminated. If the call chain forms a loop, it will also terminate execution.

The ram resource used in the contract, the payment account can be set to the following four identities:

ram_fee payer	Description
0	Contract account itself (same as _slef)
_self	Contract account itself (same as 0）
sender	Contract call account
original	The account originally calls the contract , which is used as the starting call account in the cross-contract call.

Example

contract_A --> contract_B, contract_A contract calls contract_B contract.

1. Construct an action in contract_A containing contract_B account id/account name, action name, parameters, calling account (_self), additional assets.
2. Call the send method of the action to complete the cross-contract call.

#contract_b
···
void hi(std::string name,uint64_t number)
{
    ···
}
···
#contract_A
···
struct param {
    std::string name;
    uint64_t number;
};


void inlinecall(uint64_t con_b_id, std::string con_b_name){

    param par{"hello", 100};

    // 1: When constructing an action, use the contract account id
    action contract_b_id(con_b_id, N(hi), std::move(par), _self, {1000, 1});
    contract_b_id.send();

    // 2: When constructing an action, use the contract account name
    action contract_b_name(con_b_name, N(hi), std::move(par), _self, {1000,1});
    contract_b_name.send();
}
···

Multi-index table

index

• Introduction to multi-index table
• Example
• Define_type
• Emplace
• Erase
• Find
• Modify

Introduction to multi-index table

The data must be stored in units of C++ class instances, so the stored C++ classes must be defined in the contract. Each class has a table, similar to a single table in the sql database. The difference is that it has the following characteristics:

Support for multi index

Joint indexing is not supported

Only the primary key is unique

Index type only supports uint64_t type

If you want to use a string as an index, you must use the uint64_t string_to_name(string str) in the contract library to convert the string to uint64_t. The string length is limited to 12 characters and can only include ([az].[1 -5]) a total of 32 characters

For indexes other than the primary key, when there are multiple record index values, the acquired object is the earliest inserted record.

Support for additions and deletions

example code

#include <graphenelib/contract.hpp>
#include <graphenelib/dispatcher.hpp>
#include <graphenelib/multi_index.hpp>
#include <graphenelib/print.hpp>

using namespace graphene;

class multindex : public contract
{
    struct offer;

  public:
    multindex(uint64_t id)
        : contract(id)
        , offers(_self, _self)
    {
    }

    //@abi action
    void additem(uint64_t i1, uint64_t i2, std::string name)
    {
        uint64_t pk = offers.available_primary_key();
        print("pk=", pk);
        offers.emplace(0, [&](auto &o) {
            o.id = pk;
            o.idx1 = i1;
            o.idx2 = i2;
            o.stringidx = graphenelib::string_to_name(name.c_str());
        });
    }

    //@abi action
    void getbypk(uint64_t key)
    {
        auto it = offers.find(key);
        if (it != offers.end()) {
            dump_item(*it);
        }
    }

    //@abi action
    void getbyidx1(uint64_t key)
    {
        auto idx = offers.template get_index<N(idx1)>();
        auto matched_offer_itr = idx.lower_bound(key);
        if (matched_offer_itr != idx.end()) {
            dump_item(*matched_offer_itr);
        }
    }

    //@abi action
    void getbyidx2(uint64_t key)
    {
        auto idx = offers.template get_index<N(idx2)>();
        auto matched_offer_itr = idx.lower_bound(key);
        if (matched_offer_itr != idx.end()) {
            dump_item(*matched_offer_itr);
        }
    }

    //@abi action
    void getbystring(std::string key)
    {
        auto idx = offers.template get_index<N(stringidx)>();
        auto matched_offer_itr = idx.lower_bound(N(key));
        if (matched_offer_itr != idx.end()) {
            dump_item(*matched_offer_itr);
        }
    }

  private:
    void dump_item(const offer &o)
    {
        print("offer.id:", o.id, "\n");
        print("offer.idx1:", o.idx1, "\n");
        print("offer.idx2:", o.idx2, "\n");
        graphenelib::name n;
        n.value = o.stringidx;
        print("offer.stringidx:", n.to_string().c_str(), "\n");
    }

  private:
    //@abi table offer i64
    struct offer {
        uint64_t id;
        uint64_t idx1;
        uint64_t idx2;
        uint64_t stringidx;

        uint64_t primary_key() const { return id; }

        uint64_t by_index1() const { return idx1; }

        uint64_t by_index2() const { return idx2; }

        uint64_t by_stringidx() const {return stringidx; }

        GRAPHENE_SERIALIZE(offer, (id)(idx1)(idx2)(stringidx))
    };

    typedef multi_index<N(offer), offer,
                        indexed_by<N(idx1), const_mem_fun<offer, uint64_t, &offer::by_index1>>,
                        indexed_by<N(idx2), const_mem_fun<offer, uint64_t, &offer::by_index2>>,
                        indexed_by<N(stringidx), const_mem_fun<offer, uint64_t, &offer::by_stringidx>>>
        offer_index;

    offer_index offers;
};

GRAPHENE_ABI(multindex, (additem)(getbypk)(getbyidx1)(getbyidx2)(getbystring))

Define type

private:
    //@abi table offer i64
    struct offer {
        uint64_t id;
        uint64_t idx1;
        uint64_t idx2;
        uint64_t stringidx;

        uint64_t primary_key() const { return id; }

        uint64_t by_index1() const { return idx1; }

        uint64_t by_index2() const { return idx2; }

        uint64_t by_stringidx() const {return stringidx; }

        GRAPHENE_SERIALIZE(offer, (id)(idx1)(idx2)(stringidx))
    };

    typedef multi_index<N(offer), offer,
        indexed_by<N(idx1), const_mem_fun<offer, uint64_t, &offer::by_index1>>,
        indexed_by<N(idx2), const_mem_fun<offer, uint64_t, &offer::by_index2>>,
        indexed_by<N(stringidx), const_mem_fun<offer, uint64_t, &offer::by_stringidx>>> offer_index;
    offer_index offers;

Must add comments //@abi table offer i64

offer is table name, Can't exceed 12 characters and can only be [a-z] 1-5],and can only start with letters or . .

i64 is an index type.

Struct offer{...} is a normal c++ class.

GRAPHENE_SERIALIZE(offer, (id)(idx1)(idx2)(stringidx)) is used to serialize the table such as GRAPHEN_SERIALIZE (typename, (field1)(field2)(field3)(field4)...).

uint64_t primary_key() const { return id; } Specify a unique primary key, the function name and type of this code are fixed and cannot be changed.The other three functions are used to define the secondary index.

The definition of index

typedef multi_index<N(offer), offer,  
    indexed_by<N(idx1), const_mem_fun<offer, uint64_t, &offer::by_index1>>,  
    indexed_by<N(idx2), const_mem_fun<offer, uint64_t, &offer::by_index2>>,  
    indexed_by<N(stringidx), const_mem_fun<offer, uint64_t, &offer::by_stringidx>>>  offer_index;  

`offer` is consistent with the offer in '//@abi table offer i64', `offer` is used to specify the type name previously defined

indexed_by<N(idx1), const_mem_fun<offer, uint64_t, &offer::by_index1>>,
    const_mem_fun<offer, uint64_t, &offer::by_index1>

// There are secondary indexes here and a table can define up to 16 secondary indexes.
// N(idx1) used to define the index name

In the contract constructor you need to use the contract's _self (contract id) to initialize an instance of a multi-index type.

    multindex(uint64_t id)
        : contract(id)
        , offers(_self, _self)
    {}

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Emplace

uint64_t pk = offers.available_primary_key();
print("pk=", pk);
offers.emplace(0, [&](auto &o) {
    o.id = pk;
    o.idx1 = i1;
    o.idx2 = i2;
    o.stringidx = graphenelib::string_to_name(name.c_str());
});

uint64_t pk = offers.available_primary_key() The next legal primary key used to obtain the auto-increment primary key, or you can specify it yourself

offers.emplace(0, [&](auto &o) {  
    o.id = pk;  
    o.idx1 = i1;  
    o.idx2 = i2;  
    o.stringidx = graphenelib::string_to_name(name.c_str());  
});

Inserting an object with a lambda expression to assign a value to the newly added object o o.stringidx = graphenelib::string_to_name(name.c_str()) is a way that uses a string type to query.

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Erase

Please read Find first
To delete an object by table's iterator, you should call the find function to find the iterator of the object you want to delete.

offers.erase(it) 'it' is an iterator of the object which you want find.

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Find

auto idx = offers.template get_index<N(stringidx)>();
auto matched_offer_itr = idx.lower_bound(N(key));
if (matched_offer_itr != idx.end()) {
    dump_item(*matched_offer_itr);
}

auto idx = offers.template get_index<N(stringidx)>()Get the index of the offer table name as stringidx, the offer table has 4 indexes, one is the primary key index, and the other 3 secondary indexs are idx1, idx2, stringidx

auto matched_offer_itr = idx.lower_bound(N(key))Find the object of the string key by the primary key index and return its iterator matched_offer_itr

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Modify

Modify objects by their iterators and lambda expressions

offers.modify(it, 0, [&](auto &o) {
  o.idx1 = 1000;
});

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