Calling external files (e.g. executables) in C++ in a cross-platform way

Question

I know many have asked this question before, but as far as I can see, there's no clear answer that helps C++ beginners. So, here's my question (or request if you like),

Say I'm writing a C++ code using Xcode or any text editor, and I want to use some of the tools provided in another C++ program. For instance, an executable. So, how can I call that executable file in my code?

Also, can I exploit other functions/objects/classes provided in a C++ program and use them in my C++ code via this calling technique? Or is it just executables that I can call?

I hope someone could provide a clear answer that beginners can absorb.. :p

Answer 1

So, how can I call that executable file in my code?

The easiest way is to use system() . For example, if the executable is called tool, then:

system( "tool" );

However, there are a lot of caveats with this technique. This call just asks the operating system to do something, but each operating system can understand or answer the same command differently.

For example:

system( "pause" );

...will work in Windows, stopping the exectuion, but not in other operating systems. Also, the rules regarding spaces inside the path to the file are different. Finally, even the separator bar can be different ('\\' for windows only).

And can I also exploit other functions/objects/classes... from a c++ and use them in my c++ code via this calling technique?

Not really. If you want to use clases or functions created by others, you will have to get the source code for them and compile them with your program. This is probably one of the easiest ways to do it, provided that source code is small enough.

Many times, people creates libraries, which are collections of useful classes and/or functions. If the library is distributed in binary form, then you'll need the dll file (or equivalent for other OS's), and a header file describing the classes and functions provided y the library. This is a rich source of frustration for C++ programmers, since even libraries created with different compilers in the same operating system are potentially incompatible. That's why many times libraries are distributed in source code form, with a list of instructions (a makefile or even worse) to obtain a binary version in a single file, and a header file, as described before.

This is because the C++ standard does not the low level stuff that happens inside a compiler. There are lots of implementation details that were freely left for compiler vendors to do as they wanted, possibly trying to achieve better performance. This unfortunately means that it is difficult to distribute a simple library.

Answer 2

You can call another program easily - this will start an entirely separate copy of the program. See the system() or exec() family of calls.

This is common in unix where there are lots of small programs which take an input stream of text, do something and write the output to the next program. Using these you could sort or search a set of data without having to write any more code.

On windows it's easy to start the default application for a file automatically, so you could write a pdf file and start the default app for viewing a PDF. What is harder on Windows is to control a separate giu program - unless the program has deliberately written to allow remote control (eg with com/ole on windows) then you can't control anything the user does in that program.