added c++ usage example for this driver

[BeSeeTek]

I'm using this drivers with C++ Code.

I needed some time to figure out how to use the hande to pass the instance pointer to my nonstatic member functions. An example would have helped me a lot. There for i sugest to add an c++ example to the readme like this one below and in the the pull request

2.c Using the handle to integrate this driver in c++ classes

• Non static member functions of classes need an pointer to their own instance as first argument this is implicit done by the compiler.
• This libraries pass the ctx.handle as first argument to the read and write Functions so we can store the pointer to the class instance there.
• When defining out platform member functions we must skip the first argument since this will be the this pointer See example:

class MemsSensor{
public:
  ...
private:
  int32_t platform_write(uint8_t reg, const uint8_t *bufp,uint16_t len);
  int32_t platform_read(uint8_t reg, uint8_t *bufp,uint16_t len);
  stmdev_ctx_t _dev_ctx={(stmdev_write_ptr)  &MemsSensor::platform_write,
                    (stmdev_read_ptr)    &MemsSensor::platform_read,
                        (stmdev_mdelay_ptr)  NULL, // no delay function set
                                             this};
}

• With in the classes code the driver functions can than easily be used with &_dev_ctx as first argument like:

  lsm6dsrx_device_id_get(&_dev_ctx, &_whoamI);

[ST-dot-com]

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