retrieving values from array of future objects, std::vector - c++

Question

I'm receiving an error message I have no idea how to respond to. I'm trying to retrieve the values from a vector of future objects and then output them, but I'm having trouble confirming if I'm retrieving the values.
Code:
std::vector<std::future<double>> threadResults;
int numThreads = 4;
double output;
for (int j=0; j<numThreads; j++) {
threadResults.push_back(std::async(launch::async, someCalculation, array1, array2, dimension));
}
for (auto& fut : threadResults) {
auto output = fut.get();
}
I'm new to using std::vector, so I'm unsure of how to use get() with it. Some researching brought me to the bottom for-loop, which I'm unsure if it's working. If anyone sees any obvious mistakes, please let me know, but my main concern is how to correctly use std::vector with the future class.
Here is my error message:
/tmp/ccQ19btT.o: In function `std::thread::thread<std::__future_base::_Async_state_impl<std::_Bind_simple<double (*(double*, double*, double*, int))(double*, double*, double*, int)>, double>::_Async_state_impl(std::_Bind_simple<double (*(double*, double*, double*, int))(double*, double*, double*, int)>&&)::{lambda()#1}>(std::__future_base::_Async_state_impl<std::_Bind_simple<double (*(double*, double*, double*, int))(double*, double*, double*, int)>, double>::_Async_state_impl(std::_Bind_simple<double (*(double*, double*, double*, int))(double*, double*, double*, int)>&&)::{lambda()#1}&&)':
nonOptimizedNonThreaded.cpp:(.text._ZNSt6threadC2IZNSt13__future_base17_Async_state_implISt12_Bind_simpleIFPFdPdS4_S4_iES4_S4_S4_iEEdEC4EOS8_EUlvE_IEEEOT_DpOT0_[_ZNSt6threadC5IZNSt13__future_base17_Async_state_implISt12_Bind_simpleIFPFdPdS4_S4_iES4_S4_S4_iEEdEC4EOS8_EUlvE_IEEEOT_DpOT0_]+0x2d): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
Sorry for the long horizontal scroll, let me know if there's a better to format it.
Here is how I'm compiling:
g++ threadingVector.cpp -o threadingVector -std=c++0x

Answer 1

Let me know if this isn't the right answer, and I shall delete it to avoid confusing other people, but I had the exact same issue yesterday and adding -pthread to the compiler args worked. Try:
g++ -pthread threadingVector.cpp -o threadingVector -std=c++0x
On different architectures the std::thread & company leave some of the low-level implementation of multi-threading down to the environment, and one popular library that would complete the threading implementation on linux would be pthread.