setting repeated field in protobuf using reflection

Question

I have a protobuf message similar to this:

message foo {
   repeated double values = 1;
}

I am using reflection (as explained here) to set the values like this in my C++ code

auto desc = protoMsg.GetDescription();
auto refl = protoMsg.GetReflection();
auto fd = desc->FindFieldByNumber(1); // run time value

assert(fd->is_repeated());
for (int i = 0; i < vect.size(); ++i)
   refl->SetRepeatedDouble(&protoMsg, fd, i, vect[i]);

My application is crashing inside SetRepeatedDouble function. Has anyone tried to do something like this before? (set values using reflection? Note that I have to use reflection due to dynamic nature of my application. I have simplified above code slightly not to do that for now)

Also any tips on how to possible debug this are appreciated.

Answer 1

SetRepeatedDouble() overwrites an already-present element of the array. For your code to work, the repeated field must already have a size at least equal to vect.size(), otherwise you are overrunning the array, which would indeed cause a crash (or worse). If you started with an empty message instance, then you need to add the values instead of setting them, like so:

for (int i = 0; i < vect.size(); ++i)
    refl->AddDouble(&protoMsg, fd, vect[i]);