You cannot do it with the standard fstream, you'll have to use platform specific functions.
On Windows, you can use CreateFile() or LockFileEx(). On Linux, there is flock(), lockf(), and fcntl() (as the previous commenter said).
If you are using MSVC, you can pass a third parameter to fstream's constructor. See the documentation for Visual Studio 6 or newer versions. Of course it won't work with other compilers and platforms.
Why do you want to lock others out anyway? There might be a better solution...
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