Thread safe read from a std::vector of std::tuple of double?

Question

Consider a class containing the following vector:

std::vector<std::tuple<double, double, double> > _data;

and the following member function:

inline double second(const unsigned int i) const
{
    return std::get<1>(_data[i]);
}

Do I have the guarantee that this function is thread-safe (note that I return a copy of the double)?

If not, what would be a thread-safe version of this function?

Answer 1

This is not thread-safe if the std::vector can be modified by another thread. To make it thread-safe, access to the std::vector must be synchronized. A possible solution is to introduce a std::mutex and associate it with the std::vector instance. In this case, the std::mutex would be a member variable of the class that contains the std::vector :

#include <mutex>

class X
{
private:
    std::vector<std::tuple<double, double, double>> data_;
    mutable std::mutex data_mutex_;
public:
    double second(const unsigned int i) const
    {
        // Note that 'operator[]' is not bounds checked.
        // Recommend adding a check to ensure 'i' is
        // within range or use 'at()'.

        std::lock_guard<std::mutex> lk(data_mutex_);
        return std::get<1>(data_[i]);
    }
};

Note the addition of the std::mutex makes the class non-copyable as it itself is non-copyable.