read binary file into the address of vector element

Question

I have a binary file which contains doubles of size of 8 bytes. I want to read the doubles into a vector<double>, see below

  ifstream infile("x.dat", ios::binary);
  vector<double>data(filesize/8, 0.0);
  for(int i=0; i< (filesize/8); ++i)
  {
    infile.read( (char *) (&data[i]), sizeof(double) );
  }

I know this would work if data was a C array, but not sure if this will work for vector, since vector contains more stuff than a C array(vector has methods), does the address &data[i] mean the address of the data member of the ith element?

Yes - the code's fine (apart from not checking infile for open/read errors). Why don't you run it, prove to yourself it works, and get on with the rest of the program? — Tony Delroy
– Tony Delroy, Commented May 12, 2015 at 6:08
vector is a sequential container hold data like C array with extra information like size and capacity. Yes it was also designed to be used as primitive arrays. — Fredrick Gauss
– Fredrick Gauss, Commented May 12, 2015 at 6:15
Possible duplicate of Are std::vector Elements Guaranteed To Be Continuous. — Ami Tavory
– Ami Tavory, Commented May 12, 2015 at 6:16

Mike Seymour · Accepted Answer · 2015-05-12 06:24:26Z

4

Yes, each vector element is just a double object, and &data[i] points to that object.

In fact, all the double objects are in a contiguous array, so you can read them all at once:

infile.read((char*)data.data(), filesize);

Of course, as noted below, this requires the file to have been written in a format compatible with your program's binary representation of double values, otherwise the values will be garbage.

edited May 12, 2015 at 6:24

answered May 12, 2015 at 6:15

Mike Seymour

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8 Comments

Allanqunzi Over a year ago

thanks, what does data.data() mean here? Does it return the address for the entire contiguous array of doubles?

vsoftco Over a year ago

@MikeSeymour The OP idea is in general not portable though, since the representation of doubles can vary across different OS (when dumped into files), isn't this so?

Mike Seymour Over a year ago

@vsoftco: It's as portable as the posted code. If the file was written out by a program using the same representation, then it's fine. It it used a different representation, then you'll get garbage.

Mike Seymour Over a year ago

@Allanqunzi: No, you can't compare an iterator with a pointer. But the address of the object it refers to, &*it, will be the same as &data[0], since both are the address of the same object.

Mike Seymour Over a year ago

@Allanqunzi: It rather depends on your intuition. An iterator isn't a pointer (it just behaves like one in some ways), and in general you can't compare unrelated types; from that point of view, it's to be expected that you can't compare them.

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