Tbb library: error: no match for call to function when writing custom class function instead of lambda expression

Question

I learning Thread Building Block with book "Pro TBB".

I want to rewrite to class function instead of using lambda expression as author wrote.

This is original source of the book, which I tested and it worked:

#include <iostream>
#include <tbb/tbb.h>

int main()
{
    tbb::parallel_invoke(
        [](){std::cout<<"Hello "<<std::endl;},
        [](){std::cout<<"TBB! "<<std::endl;}
    );
    return 0;
}

But when I write as follow:

#include <iostream>
#include <tbb/tbb.h>

using std::cout;
using std::endl;

void print_function(const std::string &s)
{
    cout<<s<<endl;
}
class printClass
{
private:
    const std::string &myString;
public:
    printClass(const std::string &s): myString{s} {};
    void operator()(std::string s) const{
        print_function(myString);
    } 
};

int main()
{
    std::string s1 = "Hello", s2 = "TBB!";
    tbb::parallel_invoke(
        printClass(s1),
        printClass(s2)
    );
    return 0;
}

It produced error:

In file included from /usr/local/include/tbb/tbb.h:61:0,
                 from figure_1_04_class.cpp:2:
/usr/local/include/tbb/parallel_invoke.h: In instantiation of ‘tbb::task* tbb::internal::function_invoker<function>::execute() [with function = printClass]’:
figure_1_04_class.cpp:30:1:   required from here
/usr/local/include/tbb/parallel_invoke.h:47:24: error: no match for call to ‘(const printClass) ()’
             my_function();
                        ^
figure_1_04_class.cpp:17:10: note: candidate: void printClass::operator()(std::__cxx11::string) const
     void operator()(std::string s) const{
          ^
figure_1_04_class.cpp:17:10: note:   candidate expects 1 argument, 0 provided

I follow their example in chapter 2 in the book above. This is their example and it worked too:

#include <vector>
#include <tbb/tbb.h>
#include <iostream>

using std::cout;
using std::endl;

void print_fucntion(int v)
{
    cout<<"v: "<< v<<endl;    
}
void sidebar_pfor_lambda(int N, const std::vector<int> &a)
{
    tbb::parallel_for(0, N, 1, [&a](int i)
    {
        print_fucntion(a[i]);
    });
}

int main()
{
    std::vector<int> v = {4, 5, 6, 7, 8};
    sidebar_pfor_lambda(5, v);
    return 0;
}

#include <vector>
#include <tbb/tbb.h>
#include <iostream>

using std::cout;
using std::endl;

void print_fucntion(int v)
{
    cout<<"v: "<< v<<endl;    
}

class Body
{
private:
    const std::vector<int> &myVector;
public:
    Body(const std::vector<int> &v) : myVector{v} {};
    void operator()(int i) const {
        print_fucntion(myVector[i]);
    }
};

void sidebar_pfor_function(int N, const std::vector<int> &a)
{
    tbb::parallel_for(0, N, 1, Body(a));
}

int main()
{
    std::vector<int> v = {4, 5, 6, 7, 8};
    sidebar_pfor_function(5, v);
    return 0;
}

What am I doing wrong and how to fix it?

Answer 1

tbb::parallel_invoke expects functional objects that can be invoked with zero arguments:

The expression parallel_invoke(f0,f1,...,fk) evaluates f0(), f1(), ..., fk() possibly in parallel.

Lambdas in your first example can be invoked in this way:

auto l = [](){ std::cout << "Hello" << std::endl; };
l();   // This is OK

But the functional object printClass expects one argument:

std::string s1 = "Hello";
auto l = printClass(s1);
l();   // Not OK!

The compiler complains:

candidate expects 1 argument, 0 provided

The solution is pretty simple – remove the unnecessary argument (what is it there for?):

class printClass {
    // ...
    void operator()(/* std::string s */) const {
        print_function(myString);
    }
};