C++11 cmake O3 option <no matching constructor for initialization of 'std::thread'>

Question

I use C++ with CMake makefile with -std=c++11 .

My program use several thread method. I can build and execute my program without problem.

But when I add -03 optimization flag on CMake option, I have this error message:

"no matching constructor for initialization of 'std::thread'"

First I d'ont understand why is appear only in -O3 option.

Second I would like to compile in -O3 , I see others Q&A who talk about mythread = std::thread(&X::run<A, B>, this, a, b); but it doesn't work in my program and I don't understand how to use.

Here my function I want to take into thread:

static void getPoints(Mat *in, vector<Point> *posPoint, float *h,int rad, int dex,int decy, Mat *debug = NULL );

Today I call very simply with: std::thread t1(get4points,&myImage, ...

In case of std::thread(&X::run<A, B>, this, a, b); I don't understand what is exactly &X::run<A, B> , in case of I call a function of one class in a function of the same class.

Example pseudo code:

class myclass
{
    template<int A, int B> void run(int a, int b)
    {
        // ...
    }

    void myMainfunction(int a, int b)
    {
        ?????? -> std::thread(&this::run<int, int>, this, 1, 1);
    }
};

Answer 1

From your code example you must define all your template parameters to specify the concrete function to pass in the thread constructor.

So if you have:

class myclass
{
   template<int A, int B> void run(int a, int b)
    {
    }
};

You have to specify the parameters for A & B like:

auto x = std::thread(&myclass::run<55, 66>, this, 1, 1);

If your method is static , there is no related object at all so it makes no sense to pass the object pointer to the thread constructor. You simple have to write:

class myclass
{
    template<int A, int B> static void run(int a, int b)
    {
    }
};


auto x = std::thread(&myclass::run<77, 88>, 1, 1);

You ask:

In case of std::thread(&X::run, this, a, b); I don't understand what is exactly &X::run, in case of I call a function of one class in a function of the same class.

You do not understand the difference of class and object! this points to an object of your class and not to a class itself. Start reading about the fundamentals of C++ before playing around with template stuff. In case of a static function, there is no object as already mentioned.

To get an idea if using the this pointer, the object and the call to non static functions take a look in this example:

class myclass
{   
    private:
        int ia; 

    public:
        myclass( int _ia): ia{_ia}{}

        template<int A, int B> static void staticFun(int a, int b)
        {   
            std::cout << "Val of A: " << A << " Val of B: " << B << " a: " << a << " b: " << b << std::endl;
        }   

        template<int A, int B> void Fun(int a, int b)
        {   
            std::cout << "Val of A: " << A << " Val of B: " << B << " a: " << a << " b: " << b << " ia of instance: " << ia << std::endl;
        }   
};  

int main() 
{   
    myclass mc1(1);
    myclass mc2(2);

    auto x = std::thread(&myclass::staticFun<55, 66>, 1, 2); 
    auto y = std::thread(&myclass::Fun<77,88>, &mc1, 3, 4); 
    auto z = std::thread(&myclass::Fun<78,89>, &mc2, 5, 6); 


    x.join();
    y.join();
    z.join();
}

output will be something like that:

Val of A: 55 Val of B: 66 a: 1 b: 2

Val of A: 77 Val of B: 88 a: 3 b: 4 ia of instance: 1

Val of A: 78 Val of B: 89 a: 5 b: 6 ia of instance: 2

But remember that calling the operator << of std::cout will no be synchronized at all. So each thread can write at any time into the stream and the result will be corrupted or in any order.