indexing an element from a volatile struct doesn't work in C++

May 11, 2022

I have this code:

typedef struct {
    int test;
} SensorData_t;
volatile SensorData_t sensorData[10];

SensorData_t getNextSensorData(int i)
{
    SensorData_t data = sensorData[i];
    return data;
}


int main(int argc, char** argv) {

}

It compiles with gcc version 8.3, but not with g++. Error message:

main.c: In function ‘SensorData_t getNextSensorData(int)’:
main.c:8:34: error: no matching function for call to ‘SensorData_t(volatile SensorData_t&)’
  SensorData_t data = sensorData[i];
                                  ^
main.c:3:3: note: candidate: ‘constexpr SensorData_t::SensorData_t(const SensorData_t&)’ <near match>
 } SensorData_t;
   ^~~~~~~~~~~~
main.c:3:3: note:   conversion of argument 1 would be ill-formed:
main.c:8:34: error: binding reference of type ‘const SensorData_t&’ to ‘volatile SensorData_t’ discards qualifiers
  SensorData_t data = sensorData[i];
                      ~~~~~~~~~~~~^
main.c:3:3: note: candidate: ‘constexpr SensorData_t::SensorData_t(SensorData_t&&)’ <near match>
 } SensorData_t;
   ^~~~~~~~~~~~
main.c:3:3: note:   conversion of argument 1 would be ill-formed:
main.c:8:34: error: cannot bind rvalue reference of type ‘SensorData_t&&’ to lvalue of type ‘volatile SensorData_t’
  SensorData_t data = sensorData[i];

I’m not sure if I need to add volatile as well for the data variable and the return type, shouldn’t be needed because it is copied. But I do access the sensorData array from an interrupt as well (on an embedded system), so I think I need volatile for the top level variable sensorData.

>Solution :

Your program is trying to copy a SensorData_t object. The compiler supplies a copy constructor with the following signature:

SensorData_t(const SensorData_t &)

This copy constructor will not work with volatile arguments, hence the compilation error.

You can write your own copy constructor which works with volatile SensorData_t objects (as well as non-volatile SensorData_t objects):

struct SensorData_t {
    SensorData_t() = default;

    SensorData_t(const volatile SensorData_t &other)
        : test(other.test) {
    }

    int test;
};