How to clear the content of a file after it is opened using fstream?

Question

I need to open a file in read/write mode, read its content and then clear all. So, I cannot open it in truncate mode. How can I do that?

Answer 1

Hate to disappoint you, but..

There's no standard way of clearing the contents of a file from an open std::fstream, the straight forward way is therefore to handle the two operations as what they really are.. two operations.

First handle all the reading, and later the writing (through a different stream object).

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The solution

In other words; first open the file in read-only mode (std::ifstream) and read the data you are interested in, then discard that file-handle and open the file again.. this time in write-only and truncation mode (std::ofstream), so that you will clear the contents of the file.

std::ifstream ifs ("some_file.txt");

... // read old data

ifs.close ();

std::ofstream ofs ("some_file.txt", std::ios::out | std::ios::trunc); // clear contents

... // write new data

ofs.close ();