ofstream odd behavior
c++, file-io, iostream
Solution
An `ofstream` defaults to `std::ios::trunc` -- the flag to truncate the existing content. Passing `std::ios::in` disables truncation (unless the `trunc` flag is also specified).
Actually, the rule is that `fstream` performs truncation if the `trunc` flag is used, or if the `out` flag is used and neither `in` nor `app` (notice `app` is different from `ate`, `app` repositions every write, while `ate` only affects the initial pointer). `ofstream` automatically sets `out`. `trunc` cannot be used without `out`.
Problem
I've come across an odd behavior of the ofstream, 'least odd to me. Here's my program, i'm using Visual Studio 2010 Express Edition. ``` int main () { std::ofstream file("file.txt"); file << "something1"; file.close(); file.open("file.txt", std::ios::ate | std::ios::in ); file << "something2"; file.close(); return 0; } ``` This produces the correct output. something1something2 Now if i replace the 9th line with the following code, ``` file.open("file.txt", std::ios::ate); ``` i get this output. something2 But if i replace the 9th line again, this time with this code, ``` file.open("file.txt", std::ios::ate | std::ios::in ); ``` i get this output. something1something2 Now, i guess the question is, could somebody help me out make any sense out of this? Why does the last solution work, but the middle one doesn't. EDIT: Corrected the main function. You learn something every day.