mex multithreading with c++

Hi !

I was trying to improve the speed of part of my code with a mex function. It decreased sensibly but I wante to improve it even more by using multithreading. The pure C++ function works fine, but I keep getting an error when trying to compile the mex function. I am using Matlab2019b with visual studio 2019 compiler.

I wrote the following minimal function:

#include "mex.hpp"
#include "mexAdapter.hpp"
#include <thread>
#include <chrono>
class MexFunction : public matlab::mex::Function {
public:
    void operator()(matlab::mex::ArgumentList outputs, matlab::mex::ArgumentList inputs) {
        std::thread t=std::thread(sleeponesec);
        t.join();
    }
    void sleeponesec(){
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    }
};

>> mex testmex.cpp

Building with 'Microsoft Visual C++ 2019'.

Error using mex
testmex.cpp
...\Mex\testmex.cpp(11): error C3867:
'MexFunction::sleeponesec': non-standard syntax; use '&' to create a pointer to member

Answer 1

[Matt.]

Obviously std::this_thread::sleep_for(std::chrono::milliseconds(1000)); is just an example. replacing it by : int a=2; does ntop change anything and mex test.cpp reutnrs the same error.

Answer 2

[Matt.]

I have found a workaround which consists in defining the function out of the class. I am not completely satisfied as I still do not really understand what was causing this error, but at least it works.

#include "mex.hpp"
#include "mexAdapter.hpp"
#include <thread>
#include <chrono>
void sleeponesec(){
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
}
class MexFunction : public matlab::mex::Function {
public:
    void operator()(matlab::mex::ArgumentList outputs, matlab::mex::ArgumentList inputs) {
        std::thread t=std::thread(sleeponesec);
        t.join();
    }
};

I have also tried to call

• std::thread t=std::thread(MexFunction::sleeponesec); but it returns the same error.
• std::thread t=std::thread(&MexFunction::sleeponesec); which returns the scarry following errors:

"C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\thread(40): error C2672: 'std::invoke':
no matching overloaded function found
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\thread(48): note: see reference to
function template instantiation 'unsigned int std::thread::_Invoke<_Tuple,0,1>(void *) noexcept' being compiled
        with
        [
            _Tuple=_Tuple
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\thread(56): note: see reference to
function template instantiation 'unsigned int (__cdecl *std::thread::_Get_invoke<_Tuple,0,1>(std::integer_sequence<unsigned __int64,0,1>)
noexcept)(void *)' being compiled
C:\Users\...\testmex.cpp(22): note: see reference
to function template instantiation 'std::thread::thread<void(__cdecl MexFunction::* )(matlab::data::TypedArray<double>
&),std::reference_wrapper<matlab::data::TypedArray<double>>,void>(_Fn &&,std::reference_wrapper<matlab::data::TypedArray<double>> &&)'
being compiled
        with
        [
            _Fn=void (__cdecl MexFunction::* )(matlab::data::TypedArray<double> &)
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\thread(40): error C2893: Failed to
specialize function template 'unknown-type std::invoke(_Callable &&,_Types &&...) noexcept(<expr>)'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\type_traits(1571): note: see declaration
of 'std::invoke'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\thread(40): note: With the following
template arguments:
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\thread(40): note: '_Callable=void
(__cdecl MexFunction::* )(matlab::data::TypedArray<double> &)'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\thread(40): note:
'_Types={std::reference_wrapper<matlab::data::TypedArray<double>>}'