Open a C++ file from another C++ file?

[Infernochaos]

Title seems confusing but it is not in reality .
What I'm trying to say here is that , From a C++ file (Compiled), when the user chooses the second option, I want another window that pops out and be able to convert temperature.

[Computergeek01]

You are trying to execute another PROCESS from your host. You need to ask the OS for help with this one. What OS are you running?

[Infernochaos]

My OS is Windows 7 , and once done with this topic I have another question .

Other question:
In my program, it basicaly converts celcius to farhenheit and the other way around too.
I've add some line ( I/O ) commands to put each conversion into a log.I've managed to do that but, each new conversion replace the first line only.I want to do that :

1st conversion
2nd conversion
...

[Computergeek01]

For your first question: http://msdn.microsoft.com/en-us/library/ms682425(VS.85).aspx
Don't be intimidated, it looks more complicated then it really is. Just remeber to enter the full UNC path for your 1st/2nd argument and you'll do fine. Let me know if you have trouble with it.

For your second question. You want to open the output file with the std::ios_base::app argument. This tells the compiler to append whatever data to the end of the file you're opening. See here: http://www.cplusplus.com/reference/iostream/ofstream/ofstream/

EDIT:

#include <fstream>
/* code code code */

std::ofstream outputfile(OutputFileNameAndExtention, std::ios_base::app);

The std parts are only important if you aren't using the std namespace. "OutputFileNameAndExtention" can be a string.