CODEWITHSUNDEEP
c++multithreading
ChangeMyName
ChangeMyName

Reputation: 7428

Get member data from a thread

I am reading a .csv file using multiple threads. Each thread reads a part of the .csv file, eg. thread1 reads from line:214 to line:359.

CSVReader reader1("C:\\file.csv", 214, 359);

During reading process, the data fields are stored in a vector instance.

data[i].push_back(data_field);

In main function, the codes are like below:

CSVReader reader1("C:\\file.csv", 214, 359);
CSVReader reader2("C:\\file.csv", 360, 517);

thread t1(&CSVReader::read_range, reader1);
thread t2(&CSVReader::read_range, reader2);

t1.join();
t2.join();

vector<vector<string>> temp_data = reader1.get_data();  // Here I have the problem

Ideally, reader1.get_data() should return the data between line:214 and line:359. But when I look into temp_data, I found it is not changed at all.

May I know what am I wrong? And how can I fix it?

Upvotes: 1

Views: 75

Answers (1)

Jonathan Wakely
Jonathan Wakely

Reputation: 171433

When you create the threads you pass copies of reader1 and reader2, so the copies are modified not the original objects. That means the data are added to the copies, and the original objects are unchanged. To pass them by reference use std::ref(reader1) and std::ref(reader2).

If your CSVReader::read_range function takes a reference your code should not compile, but some buggy compilers (including Visual C++) accept it.

(N.B. in general you should have shown the definition or at least the signature of CSVReader::read_range so people don't have to guess what your code does.)

Upvotes: 1

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