Why "non-standard syntax; use '&' to create a pointer to member" using thread within a CTOR?

Question

Here's the code:

#include <iostream>
#include <thread>

struct PickRandomFile {
    PickRandomFile() {
        std::thread t1(taskScanPaths);
    }

    inline void taskScanPaths() {
        // my task
    }
};

int main() {
    PickRandomFile pickRandomFile;

    return 0;
}

msvc says PickRandomFile::taskScanPaths': non-standard syntax; use '&' to create a pointer to member ThreadTrigger

What's wrong? I usually do it in gcc.

You're looking for std::thread t1(&PickRandomFile::taskScanPaths, this);. — Quentin
– Quentin, Commented Oct 22, 2019 at 13:21
Which gcc version does compile this code? All I've tried give me an error error: invalid use of non-static member function 'void PickRandomFile::taskScanPaths()' (bit less descriptive in earlier versions). online compiler — Yksisarvinen
– Yksisarvinen, Commented Oct 22, 2019 at 13:23
@markzzz In those other projects, are you using class functions or free functions? — NathanOliver
– NathanOliver, Commented Oct 22, 2019 at 13:31
@markzzz As the message says: It's non-standard syntax, i.e. something that msvc/gcc can graciously allow that would be invalid according to the C++ standard. The reasons/conditions when it allows this (or even prints a warning/errors out) are probably dependent on your compiler version and build flags (assuming that this very same code works in other circumstances). — Max Langhof
– Max Langhof, Commented Oct 22, 2019 at 13:48

463035818_is_not_an_ai · Accepted Answer · 2019-10-22 13:39:22Z

3

Free functions "decay to pointers" (similar to arrays) so

std::thread t1(taskScanPaths);

would have been ok if taskScanPaths was a free function and it would have same effect as

std::thread t1(&taskScanPaths);

However, for class member function you need the address-of to get a pointer to the member function (and you need to specify the class), as in

std::thread t1(&PickRandomFile::taskScanPaths,this);

Also note that you need to pass a object/pointer to an instance so the thread can actually call the method.

Some relevant quotes from cppreference:

Pointer to member functions

A pointer to non-static member function f which is a member of class C can be initialized with the expression &C::f exactly. Expressions such as &(C::f) or &f inside C's member function do not form pointers to member functions.

It does not explicitly mention f, but as &f does not form a pointer to member function, it is kinda safe to assume f also does not form a pointer to member function.

On the other hand:

Pointers to functions

A pointer to function can be initialized with an address of a non-member function or a static member function. Because of the function-to-pointer implicit conversion, the address-of operator is optional:

edited Oct 22, 2019 at 13:39

answered Oct 22, 2019 at 13:25

463035818_is_not_an_ai

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4 Comments

NathanOliver Over a year ago

don't forget you need a object/pointer of the class type as the second parameter if it is a non-static function.

463035818_is_not_an_ai Over a year ago

@NathanOliver thanks fixed. Anyhow I didnt pay attention to who is the asker and it seems that focus is more on "why gcc allowed it", I guess its comes as non-standard extension

NathanOliver Over a year ago

AFAIK it can't ever work. GCC can't know which object to call the function on. My guess is the OP had free functions in the other examples.

463035818_is_not_an_ai Over a year ago

@NathanOliver reading a bit I also could convince myself of the same, will add a bit more

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