如何创建从线程查询的全局可访问对象的持久列表？

Question

I've been trying to get a persistent object from a thread for hours. I want to write a shared library in C++ that starts a persistent loop in a function. In the following code snippets there is a class called Process. Process initializes a TCP/IP interface to read and write data from a Simulink model.

This is only for declaration and should not be important for this problem, but now you know what I talk about when mentioning the processes.

main.cpp

I know, it looks kinda ugly/unprofessional, but I'm fairly new to C++..

// frustrated attempt to make everything persistent
static vector<std::thread> processThreads;
static ProcessHandle processHandle;
static vector<std::promise<Process>> promiseProcess;
static vector<std::future<Process>> futureProcess;

EXPORT int initializeProcessLoop(const char *host, int port)
{
    std::promise<Process> promiseObj;
    futureProcess.push_back(std::future<Process>(promiseObj.get_future()));
    processThreads.push_back(std::thread(&ProcessHandle::addProcess, processHandle, host, port, &promiseProcess[0]));

    Process val = futureProcess[0].get();

    processHandle.handleList.push_back(val);

    return (processHandle.handleList.size() - 1);
}

ProcessHandle.cpp

The addProcess function from ProcessHandle creates the Process that should be persistent, adds it to a static vector member of ProcessHandle and passes the promise to the execution loop.

int ProcessHandle::addProcess(const char *address, int port, std::promise<Process> * promiseObj) {
    Process process(address, port);

    handleList.push_back(process);
    handleList[handleList.size() - 1].exec(promiseObj);
    return handleList.size() - 1;
}

To the main problem now...

If I change "initializeProcessLoop" to include:

    if(processHandle.handleList[0].isConnected())
    {
        processHandle.handleList[0].poll("/Compare To Constant/const");
    }

after i've pushed "val" to the processHandle.handleList everything works fine and I can poll the data as it should be.

If I instead poll it from - for examle - the main function, the loop crashes inside of the "initializeProcessLoop" because "Process val" is reassigned (?) with futureProcess[0].get().

How can I get the Process variable and the threaded loop to be consistent after the function returns?

If there are any questions to the code (and I bet there will be), feel free to ask. Thanks in advance!

PS: Obligatory "English is not my native language, please excuse any spelling errors or gibberish"...

Answer 1

Okay, first I have to declare, that the coding style above and following are by any means not best practice.

While Sam Varshavchik is still right with how to learn C++ the right way, just changing

Process val = futureProcess[0].get();

to

static Process val = futureProcess[0].get();

did the job.

To be clear: don't do this. It's a quick fix but it will backfire in the future. But I hope that it'll help anyone with a similar problem.

If anyone has a better solution (it can't get any worse, can it?), feel free to add your answer to this question.