Bill Gates: Donald Trump is not 'known for his philanthropy'

If you are a billionaire, sooner or later Bill Gates will come calling, asking you to give most of your money away to charity.

The idea, called the 'Giving Pledge' was the brainchild of Gates and his good friend Warren Buffett.

It's been so successful, Gates has even used it to mend fences with old rivals like Larry Ellison. Ellison signed the pledge years ago. Others include Microsoft co-founder Paul Allen, Mark Zuckerberg and Priscilla Chan, Hasso Plattner, Elon Musk, Ted Turner.

Along with that pledge, the billionaires usually write a letter explaining their interest in philanthropy. For instance, T. Boone Pickens wrote this funny note: "I've long stated that I enjoy making money and I enjoy giving it away. I like making money more, but giving it away is a close second."

On Wednesday Gates announced another 14 very wealthy individuals have recently signed the pledge including the founders of Airbnb, Intuit founder Scott Cook and Salesforce founder Marc Benioff. Landing Benioff was a coup. He's a huge philanthropist but he's publicly pooh-poohed this idea in favor of his own 1-1-1 philanthropy model, of giving wealth away as you earn it.

But there's one prominent wealthy guy who has not signed the pledge: Donald Trump.

Melinda and Bill Gates were asked about Trump by Re/Code's Walt Mossberg at its Code conference taking place this week in Southern California.

Melinda confirmed that Trump has not signed the pledge and Gates quipped:

"Donald hasn't been known for his philanthropy. He's been known for other things."

Melinda added, "We don't have any immediate plans to approach him."