Issue to resolve (c++ 11 concurrency)

Please consider the following scenario.

class A that holds 3 variables:

Class A
{
public:
    // ctor, getters, setters, etc...
private:
    uint8_t     x;
    uint8_t     y;
    uint16_t    z;
}

class B that holds variable of type A:

Class B
{
public:
    // probably some good stuff too...
private:
    A a;
}

There is a function running on a separate thread that modifies object a's variables: x, y and z.

And there is a function running on a gui thread that will have to read object a and display it's variables to user.

Apparently that will cause data race or some other bad things.

How to fix this scenario?

To make object a atomic? Or object a's variables: x, y and z atomic? Or both?...

Regards, Mike.