c++ no match for call to - c++

Question

I wrote the following code which gives an error;
ipt.cpp: In function ‘bool isprimet(long unsigned int, int)’:
ipt.cpp:28:86: error: no match for call to ‘(std::thread) (void (&)(long unsigned int, long unsigned int, long unsigned int, bool), const long unsigned int&, long unsigned int&, long unsigned int&, unsigned int&)’
for (unsigned long c=0;c
What am I doing wrong?
#include <iostream>
#include <thread>
#include <math.h>
using namespace std;
void ipt(const unsigned long number, const unsigned long root, const unsigned long threadid, bool &result)
{
result=true;
for (unsigned long c=5+threadid*6;c<=root;c+=(threadid+1)*6)
{
if(number % c-1 == 0) {result=false; break;};
if(number % c+1 == 0) {result=false; break;};
}
}
bool isprimet(const unsigned long number, const int nthreads)
{
if (number > 1)
{
if (number > 3)
{
if (number % 2 == 0) return false;
if (number % 3 == 0) return false;
unsigned int results[nthreads];
unsigned long root=(unsigned long)floor(sqrt(number))+1;
thread t[nthreads];
for (unsigned long c=0;c<nthreads;c++) t[c](ipt, number, root, c, results[c]);
for (unsigned long c=0;c<nthreads;c++) t[c].join();
for (unsigned long c=0;c<nthreads;c++) if (results[c]==false) return false;
return true;
}
else return true;
}
else return false;
}

Answer 1

When using std::thread, you need to send a callable object to the constructor of std::thread, and you can use Lambda expressions for this:
t[c] = new thread([&](){ ipt(number, root, c, results[c]); });
The following code works:
#include <thread>
#include <math.h>
#include <iostream>
using namespace std;
static int const MAX_THREADS = 128;
void ipt(const unsigned long number, const unsigned long root, const unsigned long threadid, bool &result)
{
result = true;
for (unsigned long c = 5 + threadid * 6; c <= root; c += (threadid + 1) * 6)
{
if (number % c - 1 == 0) { result = false; break; };
if (number % c + 1 == 0) { result = false; break; };
}
}
bool isprimet(const unsigned long number, const unsigned long nthreads)
{
if (number > 1)
{
if (number > 3)
{
if (number % 2 == 0) return false;
if (number % 3 == 0) return false;
bool results[MAX_THREADS];
unsigned long root = (unsigned long)floor(sqrt(number)) + 1;
thread* t[MAX_THREADS];
for (unsigned long c = 0; c < nthreads; c++)
t[c] = new thread([&](){ ipt(number, root, c, results[c]); });
for (unsigned long c = 0; c < nthreads; c++) {
t[c]->join();
delete t[c];
}
for (unsigned long c = 0; c < nthreads; c++)
if (results[c] == false)
return false;
return true;
}
else return true;
}
else return false;
}
int main(int argc, char *argv[])
{
for (int i = 1; i < 100; ++i)
if (isprimet(i,5))
cout << i << "\n";
return 0;
}

Answer 2

There are two problems with your code, they both occur in this line:
for (unsigned long c=0;c<nthreads;c++) t[c](ipt, number, root, c, results[c]);
which should look like this:
for (unsigned long c=0;c<nthreads;c++) t[c] = std::thread(ipt, number, root, c, std::ref(results[c]));
The first problem is the way that you are calling the function -- you can use the assignment operator as I have shown above.
Secondly, the default way that arguments are passed to a thread is by value. However, your function prototype specifies that you want to pass results[c] by reference, so you need to explicitly state that using std::ref(results[c]).
Also, you set the size of a static array with a non-const variable (so the compiler doesn't know the size at compile-time), hence all the compiler warnings. You need to use a constant to set the size, or define a global constant and use that, or pass the number of threads as a template argument, which will get rid of the compiler warnings.
Here is a live demo to a working example without warnings.