Appending content of text file to another file in C++

Question

How can you open a text file and append all its lines to another text file in C++? I find mostly solutions for separate reading from a file to a string, and writing from a string to a file. Can this elegantly be combined?

It is not always given that both files exist. There should be a bool return when accessing each of the files.

I'm sorry if this is already off-topic: Is appending text content to a file conflict-free in the meaning that multiple programs can do this simultaneously (the order of the lines DOESN'T matter) ? If not, what would be an (atomic) alternative?

possible duplicate of [Appending a new line in a file(log file) in c++](http://stackoverflow.com/questions/10071137/appending-a-new-line-in-a-filelog-file-in-c) — KevinDTimm, Oct 29 '13 at 17:24
@Kevin: That question doesn't appear to address multiple simultaneous writers. — Ben Voigt, Oct 29 '13 at 18:16
From what I tested there are no conflicts. I mark this thread as answered. — fotinsky, Nov 02 '13 at 00:05

score 8 · Accepted Answer · edited Apr 05 '18 at 23:30

8

I can only speak for opening a file and appending it to another file:

std::ifstream ifile("first_file.txt");
std::ofstream ofile("second_file.txt", std::ios::app);

//check to see that the input file exists:
if (!ifile.is_open()) {
    //file not open (i.e. not found, access denied, etc). Print an error message or do something else...
}
//check to see that the output file exists:
else if (!ofile.is_open()) {
    //file not open (i.e. not created, access denied, etc). Print an error message or do something else...
}
else {
    ofile << ifile.rdbuf();
    //then add more lines to the file if need be...
}

REFERENCES:

http://www.cplusplus.com/doc/tutorial/files/

https://stackoverflow.com/a/10195497/866930

edited Apr 05 '18 at 23:30

Remy Lebeau

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answered Oct 29 '13 at 17:53

jrd1

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Please don't forget to close the streams eventually. – crisron Dec 19 '16 at 10:06
2

@crisron they close automatically when they go out of scope – Remy Lebeau Apr 05 '18 at 23:30

score 1 · Answer 2 · answered Oct 29 '13 at 17:43

1

std::ifstream in("in.txt");
std::ofstream out("out.txt", std::ios_base::out | std::ios_base::app);

for (std::string str; std::getline(in, str); )
{
    out << str;
}

answered Oct 29 '13 at 17:43

David G

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Appending content of text file to another file in C++

2 Answers2

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