CODEWITHSUNDEEP
c++protocol-buffersmemcpy
akristmann
akristmann

Reputation: 444

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

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.

Upvotes: 27

Views: 60488

Answers (5)

D&#225;vid T&#243;th
D&#225;vid T&#243;th

Reputation: 3235

As an alternative to the excellent answer posted by @mgild, Assign can also be used in this situation, so the data would not be copied to a temporary before being moved.

fMessage.mutable_samples()->Assign(fData.begin(), fData.end())

Credit to Marek R's answer

Upvotes: 2

Azmisov
Azmisov

Reputation: 7253

@mgild's answer will implicitly call the RepeatedField(Iter begin, Iter end) constructor to create a temporary to be move assigned. Same with @nazgul's answer, which explicitly creates a temporary RepeatedField and swaps it.

Much simpler, avoiding the creation of a new object would be:

fMessage.mutable_samples()->Add(fData.begin(), fData.end())

If samples field is not empty already, you can call Clear method prior.

Internally, this uses std::copy (so long as fData is a forward iterator), so will be just as fast as any memcpy implementation you come up with.

Upvotes: 2

nazgul
nazgul

Reputation: 519

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 faster than yours since it avoids initial iteration and setting values to 0.

Upvotes: 23

Arthur George
Arthur George

Reputation: 43

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!

Upvotes: -3

mgild
mgild

Reputation: 794

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

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

Upvotes: 57

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