Copy a std::vector to a repeated field from protobuf with memcpy

问题

At first I have this simple protobuf file

message messagetest
{
    ...
    repeated float samples = 6;
    ....
}

Which creates a headerfile with this methods

    //repeated float samples = 6;
      inline int samples_size() const;
      inline void clear_samples();
      static const int kSamplesFieldNumber = 6;
      inline float samples(int index) const;
      inline void set_samples(int index, float value);
      inline void add_samples(float value);
      inline const ::google::protobuf::RepeatedField< float >&  samples() const;
      inline ::google::protobuf::RepeatedField< float >* mutable_samples();

What I'm basically doing is to copy all data one by one in a for loop.

int main(int argc, char** argv)
{    
    messagetest fMessage;

    vector<float> fData (1000, 0);

    // Create 1000 random values
    for (int i = 0; i < fData.size(); i++)
    {
        fData[i] = rand() % 1001;
    }

    for (int j = 0; j < fData.size(); j++)
    {
        fMessage.add_samples(fData[j]);    
    }

    return 0;
}

But I want to use a method like memcpy to accelerate the copy process. It is just an idea that comes to my mind. If it's completely wrong correct me. The last declaration in the headerfile is:

inline ::google::protobuf::RepeatedField< float >* mutable_samples();

I have no idea what this method does (lack of skill). But it kind of looks like a vector. Maybe that's the solution for my problem. If so, I have no idea how to implement it.

Edit SOLVED:

Memcpy is in any case faster, because you have one method call and then you copy the data. Compared to the for loop which has to call the "fMessage.add_samples()" method 1000 times.

One flaw of this approach is that you have to know the size of your vector to reserve data for your samples, but then you can allocate at programm start the memory for fMessage.sample (this works for me).

fMessage.mutable_samples()->Reserve(fData.size());

for (int j = 0; j < fData.size(); j++)
{
   fMessage.add_samples(0);          
}

And now i can get the data from a stream in a while loop and copy them with memcpy to my datastructer

while(42)
{
   //fancy streaming things to get vector fData

  memcpy(fMessage.mutable_samples()->mutable_data(),
         &fData[0],
         sizeof(float)*fData.size());
}

回答1:

Since this isn't here yet and I like one-liners:

*fMessage.mutable_samples() = {fData.begin(), fData.end()};

回答2:

I found the shortest way to copy vector into repeated field as this:

google::protobuf::RepeatedField<float> data(fData.begin(), fData.end());
fMessage.mutable_samples()->Swap(&data);

it is probably also the faster than yours, since it avoids initial iteration and setting values to 0

回答3:

fMessage.mutable_samples()

return an array of pointer of samples : [*sample1, *sample2, sample3, ...].

&fData[0]

is the address of first element of fData.

memcpy(fMessage.mutable_samples()->mutable_data(),
     &fData[0],
     sizeof(float)*fData.size());

So I do not think the code above can successfully fill data from fData to fMessage. It's totally wrong!

来源：https://stackoverflow.com/questions/15499641/copy-a-stdvector-to-a-repeated-field-from-protobuf-with-memcpy

标签

c++

protocol-buffers

memcpy