I am trying to encode some accelerometer data using nanopb, but this error pops up no matter which data type I try to encode:

no match for 'operator=' (operand types are 'pb_callback_t {aka pb_callback_s}' and 'String')

I have tried with ints and strings with the same result, only with ints I get Int instead of String in the error message. My proto file looks like this:

syntax = "proto2";
message accelMessage {
    repeated string values = 1;
}

and my code on Arduino is:

#include "pb_encode.h"
#include "accel.pb.h"


String accelString;
uint8_t buffer[120];
size_t message_length;
bool status;
_accelMessage message = accelMessage_init_zero;
/* Create a stream that will write to our buffer. */
pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer));
message.values = accelString;

status = pb_encode(&stream, accelMessage_fields, &message);
message_length = stream.bytes_written;

/* Then just check for any errors.. */
if (!status)
{
    printf("Encoding failed: %s\n", PB_GET_ERROR(&stream));
    return 1;
}

The error is raised on the line

message.values = accelString;

UPDATE: According to nanopb docs i should have something like

typedef struct {
   int32_t value;
} Example; 

instead i get

typedef struct _accelMessage {
    pb_callback_t values;
/* @@protoc_insertion_point(struct:accelMessage) */
} accelMessage;

in my accel.pb.h file generated by protoc

  • The message.values obviously expects some pb_callback_t callback (union over encode/decode function pointer) and not some empty String. – KIIV Nov 30 at 10:27
  • @KIIV The string in question is not empty it has accelerometer values in it. Can you please give me some pointers as to how to encode this string properly? – m.umar Nov 30 at 11:20
  • Hint: Avoid using the Arduino String class, and instead use a c-string. – Mikael Patel Nov 30 at 12:06

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