CODEWITHSUNDEEP
c++binaryfstreamifstreamofstream
NewCoder
NewCoder

Reputation: 23

c++ how modify file using fstream

I want to edit the first 100 characters of a file, I do this, but the new characters override the previous ones (like the photo) click here photo

my code :

fstream fileStreamIn("text.txt", ios::in | ios::out | ios::binary);

int theSize = 100;
string theMainBuffer(theSize, '\0');
fileStreamIn.read(&theMainBuffer.front(), theSize);
theMainBuffer.resize(fileStreamIn.gcount());
//cout << theMainBuffer << endl;
fileStreamIn.close();


fileStreamIn.open("text.txt", ios::in | ios::out | ios::binary);
fileStreamIn  << "blahblah ";
fileStreamIn.close();

I want "blahblah" to be added to the contents of the file and the previous contents of "helloworld" not to be deleted

output :

blahblahrld !
è !©ª}2•¼Ü²ù­XkLÉ·ð„!ð–ç„ñWïðʃ¡ åñ·§Dß}ˆ¹mÐÕŠw:—*ËtMÒJf-Öù“hñ<³:rÛä‡   ”‘Ôyv-4mXþeߧzè’¬ŒŽ<¤‘“‰l'g‚Šâ¡;¬Èa|ÔÁ3îú€;‰±Ï.ÖLáÑȽ[ïÿÿúU%ã2§Ls§n~çˆÏÔäÔ™ 4øÒ‘Ö°,y•»Ô'`` ¬ÜgÜò`÷Tº^E1ØàùÛ÷i§d¨Ù`I5»7á8Zéz0¥Ž’3Y7Êœ¦}eíÝΦIm?óbÙOâ-ŸäëŠgýhýR
Â3‘†y±è±/VŠ¤?Ïù4?’ÑûIÆLQ~DãŠ?Ôêð#N ]³böPK     ZQamë  š  PK      5   -

I want this output :

blahblah hello world !
è !©ª}2•¼Ü²ù­XkLÉ·ð„!ð–ç„ñWïðʃ¡ åñ·§Dß}ˆ¹mÐÕŠw:—*ËtMÒJf-Öù“hñ<³:rÛä‡   ”‘Ôyv-4mXþeߧzè’¬ŒŽ<¤‘“‰l'g‚Šâ¡;¬Èa|ÔÁ3îú€;‰±Ï.ÖLáÑȽ[ïÿÿúU%ã2§Ls§n~çˆÏÔäÔ™ 4øÒ‘Ö°,y•»Ô'`` ¬ÜgÜò`÷Tº^E1ØàùÛ÷i§d¨Ù`I5»7á8Zéz0¥Ž’3Y7Êœ¦}eíÝΦIm?óbÙOâ-ŸäëŠgýhýR
Â3‘†y±è±/VŠ¤?Ïù4?’ÑûIÆLQ~DãŠ?Ôêð#N ]³böPK     ZQamë  š  PK      5   -

What is the problem, how can I solve the problem? thanks

Upvotes: 1

Views: 2125

Answers (2)

John Park
John Park

Reputation: 1764

If you don't care to keep the first 100 bytes, simply create 100 lengths of string, change some values and write it to the stream would be enough. Reading a file is not needed.

std::fstream fs("text.txt", ios_base::out | ios_base::binary);

string buffer(100, ' ');
string update="Hello";
buffer.replace(0, update.size(), update);

fs.seekp(20);   // move to write position
fs.write(buffer.data(), buffer.size());

fs.close();

Upvotes: 1

Sagar Mahour
Sagar Mahour

Reputation: 65

Use ios::trunc as the file open mode. For more info check out this.

Upvotes: 0

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