C ++ OpenMP和std :: vector <bool>

Question

Why does the following code not work (as excpected) for std::vector of bool ? Some elements are randomly false in the bool-vector. For the int vector all works fine (tested with many more than just 3 loops).

I am on ubuntu 14.04 64bit with g++ 4.8.4.

#include <iostream>
#include <vector>

using namespace std;


class TestBool
{
public:
    TestBool() {}
    bool test() {return true;}
    int testInt() {return 10;}
};
void testBVec(vector<bool> &bv, size_t loop)
{
    cout << "boolvec loop " << loop << endl;
    for(size_t i = 0; i < bv.size(); i++) {
        if( ! bv[i])
            cout << "wholy shit with bool at index " << i << endl;
    }
}
void testIntVec(vector<int> &iv, size_t loop)
{
    cout << "intVec loop " << loop << endl;
    for(size_t i = 0; i < iv.size(); i++) {
        if( iv[i] != 10)
            cout << "wholy shit with int at index " << i << endl;
    }
}


int main()
{
    vector<TestBool> tv(10);
    size_t loops = 3;

    for(size_t i = 0; i < loops; i++ ) {
        vector<bool> bv(10);
        vector<int> iv(10);

        #pragma omp parallel for
        for(int j = 0; j < 10; ++j) {
            bv[j] = tv[j].test();
            iv[j] = tv[j].testInt();
        }
        testBVec(bv, i+1);
        testIntVec(iv, i+1);
    }

    return 0;
}

Answer 1

vector<bool> is the explicit template specialization which packs boolean values into bits of some integer variables. To make the bits writable by semantically-correct subscript, vector<bool>::operator[] returns some proxy object which is convertible to bool and has assignment operator for bool. The proxy objects for different indices may reference the same underlying memory, so simultaneous access to different bits is not guaranteed to be thread-safe.

For a competent proof that it's not a compiler bug, see'Data races' section of this page:

Simultaneous access to different elements is not guaranteed to be thread-safe (as storage bytes may be shared by multiple bits).

or Section 23.2.2 of the C++ Standard :

2. <...> implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence, excepting vector<bool> , are modified concurrently.
3. <...> As an exception to the general rule, for a vector<bool> y , y[0] = true may race with y[1] = true .

As @Anton advised, use a type other than bool .

Answer 2

Most probably it's because vector<bool> transformed to array of bits by compiler. Just use vector<int> or vector<char> storing 0s and 1s into it, if you don't need bit array.

Answer 3

In addition to Sergey and Anton answers, let me point out that vector is known to be not thread-safe as described in the Data Races section in http://www.cplusplus.com/reference/vector/vector-bool .

Your alternatives are (as others suggested) either go to vector of other types or protect the statement

bv[j] = tv[j].test();

with #pragma omp critical .