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I'm currently looking at producing a C++ library. I've not much experience with C++ and have what is probably a very basic question about class instance method calling.

main.cpp

msgserver m;
std::thread t1(m.startServer, "192.168.50.128", 8081);

msgserver.h

class msgserver
{
public:    
    msgserver() { }
    int startServer(std::string addr, int port);
};

msgserver.cpp

int msgserver::startServer(string addr, int port)

This code results in:

[C3867] 'msgserver::startServer': non-standard syntax; use '&' to create a pointer to member

I know i can fix the compiler error by making this method static but I'm unsure if that is a requirement imposed by the fact it's being called in a thread constructor (which doesn't allow the parens on the call signature) or if I need to figure out the syntax.

I've read around this and it's actually left me a bit more confused that when I started. It seems any fix I apply like:

int &msgserver::startServer(string addr, int port)
or
std::thread t1(&m.startServer, "192.168.50.128", 8081);

Is actually illegal syntax according to the compiler.

How can I call this as an instance method? Or is actually a good idea to start a thread running in the background on a static function?

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14
  • I'm not using any textbook. I'm guessing you're going to tell me that's my first mistake ... ;) Commented Jul 11, 2022 at 16:33
  • 2
    please show a minimal reproducible example Commented Jul 11, 2022 at 16:34
  • 2
    std::thread t1(&msgserver::startServer, m, "<ip>", 8081); Commented Jul 11, 2022 at 16:36
  • 3
    @Jammer I've got 25 years desktop dev behind me in other languages which helps -- One huge warning -- do not use any of those languages as a model in writing C++ code. If you use any of those languages as a model, you will wind up with 1) Buggy code, 2) Inefficient code, 3) Code that looks weird to a C++ programmer when trying to make C++ look like language <X>'s way of doing things. C++ has to be learned as if none of those other languages exist. For 3), I've seen int y = 10; string x = "" + y;, which looks ordinary to a Java programmer, but is totally wrong in C++ (concatenating). Commented Jul 11, 2022 at 16:39
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    @Jammer That's exactly what it does. You would be surprised how many Java programmers just do this, because that's what they're used to seeing. Now imagine a Java programmer trying to concatenate an int onto a string in C++, and they write that code. Then they can't understand why their C++ code doesn't work. But a C++ programmer looking at that same code would see it looks totally alien, weird, and just plain nuts. That is the perfect example of using another language as a model in writing C++ code, and totally failing. Commented Jul 11, 2022 at 19:43

2 Answers 2

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3

The syntax for a getting a pointer to a member function is &<class name>::<function_name>.

In this case &msgserver::startServer would be the correct expression. Since std::invoke is used on the background thread, you need to pass the object to call the function for as second constructor parameter for std::thread, either wrapped in a std::reference_wrapper or by passing a pointer:

std::thread t1(&msgserver::startServer, std::ref(m), "192.168.50.128", 8081);

or

std::thread t1(&msgserver::startServer, &m, "192.168.50.128", 8081);
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Comments

1

Replace

msgserver m;
std::thread t1(m.startServer, "192.168.50.128", 8081);

with the lambda function

msgserver m;
std::thread t1([=](std::string addr, int port){ m.startServer(addr, port); }, 
    "192.168.50.128", 8081);

I'm guess that you expected your version to do what the lambda function does by some kind of C++ magic. But that's not how C++ works.

Completely endorse the recommendation that you get a C++ book. Now you have lambda functions to add to your list of topics to learn.

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7 Comments

Ahhhh lambdas. Thank you. Will go study and test.
Missing a closing "}"
@Jammer Yes, fixed now
Ahhh, fixed. Fab. Thank you, will go check.
@Jammer This lambda function copies the msgserver object, that may or may not be a good idea. If not then replace [=] with [&] but then you have the new issue that the lifetime of your thread and the lifetime of your msgserver are no longer linked.
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