When std:: bind meets this

background

Read the code of previous projects of the company , See a code as follows ：

        if (!m_api->SubFutureL1(
                m_config.config.symbol[0].future, std::bind(
                                                      &ArbitrageFc::OnSymbol1FutureMd, this,
                                                      std::placeholders::_1)))

among ,SubFutureL1 and OnFutureL1MdFunc Is defined as follows ：

bool StrategyEngineImp::SubFutureL1(int64_t symbol, OnFutureL1MdFunc callback) {
    
  m_future_l1_callback[symbol] = callback;
  return m_mm.SubFuture(symbol);
}

typedef std::function<void(const FutureL1MarketData&)> OnFutureL1MdFunc; //std::function  Next time 

void OnSymbol1FutureMd(const FutureL1MarketData& md); //（ class ArbitrageFc Member function of ）

About the “bind + this” The way is not understood , After checking , Special records ;

std::bind

origin

stay c++98 in , Yes std::bind1st and std::bind2nd, Can be used to bind func Of 2 Parameters , But the use is not friendly ; later c++11 Introduced in the std::bind, Yes, before bind 1st and 2nd With the essential improvement ; Reference resources C++11 Medium std::bind The saying in is ：
bind In itself, it is an idea of delayed calculation , It can bind ordinary functions 、 Global function 、 Static functions 、 Class static functions and even class member functions

Example

Let's look directly at specific examples （ Also refer to the above link ）：

#include <iostream>
#include <functional>
using namespace std;

int TestFunc(int a, char c, float f)
{
    
    cout << a << endl;
    cout << c << endl;
    cout << f << endl;

    return a;
}

int main()
{
    
    auto bindFunc1 = bind(TestFunc, std::placeholders::_1, 'A', 100.1);
    bindFunc1(10);

    cout << "=================================\n";
    // hold TestFunc Bound to the bindFunc2 On ,bindFunc2 The second parameter of is TestFunc The first parameter of 
    //bindFunc2 The first parameter of is TestFunc Second parameter of , The last parameter is fixed to 100.1
    // Be similar to  TestFunc(bindFunc2's_var_2, bindFunc2's_var_1, 100.1);
    auto bindFunc2 = bind(TestFunc, std::placeholders::_2, std::placeholders::_1, 100.1);
    bindFunc2('B', 10);

    cout << "=================================\n";

    auto bindFunc3 = bind(TestFunc, std::placeholders::_2, std::placeholders::_3, std::placeholders::_1);
    bindFunc3(100.1, 30, 'C');

    return 0;
}

use g++ Execute after compilation , give the result as follows ：

As you can see from the code above ,bind It can bind some parameters at the same time , Parameters that are not provided are represented by placeholders , Then pass in the actual parameter value at run time .PS： The bound parameters will be passed to the specific function in the way of value passing , Placeholders will be passed by reference . as everyone knows , Static member functions can actually be regarded as global functions , Non static member functions need to be passed this Pointer as the first parameter , therefore std::bind Can easily bind member functions .

Combining theory with practice

For my question , What is more useful is Non static member functions need to be passed this Pointer as the first parameter
Answer my questions ：

m_api->SubFutureL1(
                m_config.config.symbol[0].future,
                std::bind(&ArbitrageFc::OnSymbol1FutureMd, this, std::placeholders::_1));

It can be disassembled as follows ：
1、bind relevant ：

OnFutureL1MdFunc callback = std::bind(&ArbitrageFc::OnSymbol1FutureMd, 
this, std::placeholders::_1);

Finally, it is equivalent to the effect ：

callback = this->OnSymbol1FutureMd(const FutureL1MarketData& var); 
//OnSymbol1FutureMd The first parameter of , Namely callback  The first parameter of 

In function SubFutureL1 Implemented callback binding , Then when there is a callback , That's what's called ：

OnSymbol1FutureMd(var);
//var Namely “FutureL1MarketData& md”

And this md How did it come about ？ It involves

//  Futures market 
    while (m_mm.GetFutureL1Seq() > future_l1_seq) {
    
      auto md = m_mm.GetFutureL1Market(++future_l1_seq);
      if (md) {
    
        auto iter = m_future_l1_callback.find(md->symbol);
        if (iter != m_future_l1_callback.end()) {
    
          tlogi(" Futures market ")
            << utils::symbol::ToString(md->symbol) << " " << md->update_time
            << " " << md->bid_price[0] << " " << md->bid_volume[0]
            << " " << md->ask_price[0] << " " << md->ask_volume[0]
            << " " << md->last_price << " " << md->volume;
          iter->second(*md);
        }
      }
    }

Among them iter->second(*md); This callback is implemented ;

Business understanding

Let's talk about the process （ This is strongly related to business , And you also need other code contexts , I guess I can only make a record for myself ）：
1、 Complete the binding of quotation callback in initialization ; Initialization is from multiple levels , Two aspects ：

class StrategyEngineImp : public StrategyEngine–>
member ：market::MarketManager& m_mm–>
member ：
xytp::market::client::XyMarketClient* m_fm_master = nullptr; and
xytp::market::client::XyMarketClientCallback* m_fm_master_cb = nullptr;
because ：
m_fm_master =
xytp::market::client::XyMarketClient::Create(m_config.future_master_lib.c_str());
also
xytp::market::client::XyMarketClient* Create() {
return &market::redis::RedisMarketFuture::Instance();
}
therefore ：
m_fm_master It's actually a RedisMarketFuture Example , stay
MarketManager::InitMarket() In the call ：m_fm_master->Init
That is to say bool RedisMarketFuture::Init(const std::string& config) in ：
m_redis->SetSubCallback(std::bind(
&RedisMarketFuture::OnPub,
this,
std::placeholders::_1,
std::placeholders::_2))
So as to achieve Pub Callback binding for ：
So when the market comes , The quotation gateway receives ？ And then through Redis Release , At this time ,StrategyEngineImp The members of ：market::MarketManager& m_mm Inside m_fm_master and m_fm_master_cb , It's going to come back m_fm_master_cb , That is to say cb Member functions in ：OnFutureMarket, So as to analyze the transmitted market to m_fm_master_cb The members of MarketManager& m_mm Inside market::FutureL1MarketData* future_md = m_mm.GetFutureL1Market(seq),
The whole relationship ：
StrategyEngineImp–》market::MarketManager& m_mm;–》FutureMarketCallback–》m_mm(MarketManager::Instance())–》market::FutureL1MarketData* future_md = m_mm.GetFutureL1Market(seq);
Above 2 Members of classes m_mm In fact, it refers to the same object instance 【market::MarketManager::Instance()】

2、 stay StrategyTradingEngine::MainThread in , When the market comes , Will be based on seq To judge and know , Then from the previous callback binding , Get the latest market , And then through iter->second(*md); To achieve market driven

More learning references

C++11 Medium std::bind
C++11 Medium std::function