Q:

How to truncate a file while it is open with fstream

I know it is possible to truncate a file with
std::fstream fs(mypath, std::fstream::out | std::fstream::trunc);

but I need to read the file, truncate it, then write new contents all with the same file handle (so the whole operation is atomic). Anyone?

A:

I don't think you can get "atomic" operation but using the Filesystem Technical Specification that has now been accepted as part of the Standard Library (C++17) you can resize the file like this:
#include <fstream>
#include <sstream>
#include <iostream>
#include <experimental/filesystem> // compilers that support the TS
// #include <filesystem> // C++17 compilers

// for readability
namespace fs = std::experimental::filesystem;

int main(int, char*[])
{
    fs::path filename = "test.txt";

    std::fstream file(filename);

    if(!file)
    {
        std::cerr << "Error opening file: " << filename << '\n';
        return EXIT_FAILURE;
    }

    // display current contents
    std::stringstream ss;
    ss << file.rdbuf();
    std::cout << ss.str() << '\n';

    // truncate file
    fs::resize_file(filename, 0);
    file.seekp(0);

    // write new stuff
    file << "new data";
}

A:

File streams don't support truncation except when opening a file. Also, the operations wouldn't be "atomic" anyway: at least, on POSIX systems you can happily read and write a file already opened by another process.

