BeOS C++ Development
Introduction
For C++ development on BeOS, you'll probably be using GNU C/C++ compiler. When you setup BeOS from a purchased CD, you'll have the option of installing the development tools. Those tools include the BeOS port of GNU C/C++ compiler. If you downloaded the Free Personal Edition, then you'll have to download those tools seperately.
A console on BeOS is called a "terminal". You will find it listed under "Applications" in your BeMenu:

Obtainment
If you don't already have the BeOS development tools you can download them from ftp://ftp.be.com/pub/beos/BeOS5-DevTools.zip.
Installation
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Download ftp://ftp.be.com/pub/beos/BeOS5-DevTools.zip.

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Open up
BeOS5-DevTools.zip
by double-clicking it. You should see the expander program.

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You can see the contents of the file by marking the "Show Contents" checkbox.

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Make sure the "Destination" text is set to be a single forward slash (i.e. "root").

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Click the "Expand" button and wait for it to complete.
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Open up a "Terminal" window. This should be on your BeMenu under "Applications".

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Type the following:
gcc -v
[enter]
You should get the output:
Reading specs from /boot/develop/tools/gnupro/lib/gcc-lib/i586-beos/2.9-beos-991026/specs
gcc version 2.9-beos-991026
Build Program: From Single File
See GNU C/C++ for Linux.
Links
GNU C/C++ for Linux