How to determine when a file is updated in c++?

I am using an application which takes File X as input and gives File Y and Z as output (graphs and plots)

Now if the applicatoin is run again :

1. File Y and Z already exists and File X is not updated then it should not be over-written.
2. File X is updated, then File Y and Z must be overwritten.

The code is written in c++, how can i change the code such 开发者_如何学Pythonthat both the conditions are met? Should I take time stamp of X everytime?

If you are on a POSIX machine, use stat or a related function such as fstat. The windows equivalents are described at http://msdn.microsoft.com/en-us/library/14h5k7ff(v=vs.80).aspx.

The structure returned by stat or the windows counterparts have a field that represents the time at which the file was last modified.

For a portable solution, Boost.Filesystem has last_write_time that returns the time of last modification as a std::time_t.

You could just keep a hash of file X. If the has changes, the file has been changed (updated), and you will need to update Y and Z.

if (XHashPrev != XHashNow || !YZExist)
    // update files.

using David Hammen's answer, compare modification timestamp of X against Y and Z, if X was modified later than Y or Z - so they are outdated and should be rewritten

Yes, you should inspect the file X and see (two options):

• if its update date changed or
• or if its content changed (using a CRC32 as an example).

The first is the easiest and the fastest. The second is the most accurate.
EDIT further to the comments below: These two options can be combined to reach a compromise.

Another possibility: write the basic program with X always overwriting Y and Z. Invoke the basic program with make. A makefile that looks like:

Y Z : X
    basicProgram X Y Z

should do the trick. You can then invoke it with either "make Y" or "make Z" Bottom line: let make worry about timestamps and such.

Please note that when checking for last update time, there are some limitations:

... The [time] resolution is as low as one hour on some filesystems... During program execution, the system clock may be set to a new value by some other, possibly automatic, process ...