fmt::output_file doesn't erase contents of existing file

[bluebear94]

I'm not sure what the intended behavior is, but fmt::output_file, when opened on an existing file, does not clear its contents. That is, given this code:

auto out = fmt::output_file("data/test.txt");
out.print("Hello World!\n");

and this text in data/test.txt:

This should be overwritten completely!

the file looks like this after running the code:

Hello World!
e overwritten completely!

I believe this behavior is not intuitive and can cause subtle bugs; therefore, the default should be changed to clear all the contents, or at least it should be documented.

• fmt version: 7.1.2
• OS: Linux

[vitaut]

You can pass the usual open flags to file_open, e.g.

 auto out = fmt::output_file("test.txt", O_WRONLY | O_TRUNC);

A PR to switch the default to truncate is welcome.

[vitaut]

Switched to truncation by default in 119f7dc.