Error copying and pasting data from a file to another - c++

Question

I am writing a code to merge multiple text files and output a single file.
There can be up to 22 input text files which contain 1400 lines each.
Each line has 8 bits of binary and the new line characters \n.
I am out putting a single file that has all 22 text files merged.
Problem is with my output file, after 1400 lines it appears that the content from the previous file is still being placed into output file(although the length of the previous file was 1400 lines). This extra content also begins to have additional line space between each row if opened by microsoft office or sublime, however it is interpreted as a single line if opened by notepad or excel(a single cell in excel).
Following is the picture of expected behaviour of the output file,
Here is a picture of abnormal behaviour. This starts when the first file finishes.
I know this data is from the first file still because the second file starts from 00000000
And here is the start of the second file,
And this abnormal behavior repeats every single time the files are switching.
My implementation to achieve this is as follows:
repeat:
if(user_input == 'y')
{
fstream data_out ("data.txt",fstream::out);
for(int i = 0; i<files_found; i++)
{
fstream data_in ((file_names[i].c_str()),fstream::in);
if(data_in.is_open())
{
data_in.seekg(0,data_in.end);
long size = data_in.tellg();
data_in.seekg(0,data_in.beg);
char * buffer = new char[size];
cout << size;
data_in.read(buffer,size);
data_out.write(buffer,size);
delete[] buffer;
}else
{
cout << "Unexpected error";
return 1;
}
data_in.close();
}
data_out.close();
}else if(user_input == 'n')
{
return 1;
}else
{
cout << "Input not recognised. Type y for Yes, and n for No";
cin >> user_input;
goto repeat;
}
Further information:
I have checked the size variable and it is as I expect, 14000.
8 bits, and a \ with n = 10 characters per line,
1400 rows x 10 = 14000.
Assuming reader of code to be experienced.

Answer 1

Sorry to bump this question, but I really like question that are marked as answered. JoachimPileborg answer seems to have worked for you:
Also, instead of seeking and checking sizes and allocating memory, why
not just do e.g. data_out << data_in.rdbuf();? This will copy the
whole input file to the output. – Joachim Pileborg Jul 29 at 17:26
A reference http://www.cplusplus.com/reference/ios/ios/rdbuf/ and an example:
#include <fstream>
#include <string>
#include <vector>
int main(int argc, char** argv)
{
typedef std::vector<std::string> Filenames;
Filenames vecFilenames;
// Populate the list of file names
vecFilenames.push_back("Text1.txt");
vecFilenames.push_back("Text2.txt");
vecFilenames.push_back("Text3.txt");
// Merge the files into Output.txt
std::ofstream fpOutput("Output.txt");
for (Filenames::iterator it = vecFilenames.begin();
it != vecFilenames.end(); ++it)
{
std::ifstream fpInput(it->c_str());
fpOutput << fpInput.rdbuf();
fpInput.close();
}
fpOutput.close();
return 0;
}