在boost lambda中使用三元运算符

Question

如何执行以下操作？

vector<int> vec(100);
boost::iota(vec, 0);
static int counter = 0;
boost::for_each(vec, 
    cout << constant(left) << constant(setw(5)) 
         << _1 * 2 << (++var(counter) % 5 == 0 ? "\n" : " "));

我的错误信息是：

error: value of type '...'  is not contextually convertible to 'bool'