问题:

I am looking for an alternative to C-style union. boost::variant is one such option. Is there anything in std C++ ?

union {
   int i;
   double d;
}

回答1:

As several commenters said: No, there is no Boost Variant-alike in standard C++. Maybe in a few years there will be, but why wait--use Boost Variant today!

Edit (four years later, 2016): In C++17 there will be std::variant. Similar but not identical to boost::variant. So when your compiler supports C++17, you will have a solution in the standard library.

回答2:

A few years passed: Now a proposal is on the way. Possibly boost::variant will make it into C++17! Until then we will have to live with boost::variant which is still awesome. Note that there will be some changes in the details in std::variant.

UPDATE: The proposal made it. std::variant will be part of the new C++17 standard. Here's the June 2016 meeting report by Herb Sutter. There he confirms it.

回答3:

std::variant is now officially going to be a part of the C++17 standard library! https://herbsutter.com/2016/06/30/trip-report-summer-iso-c-standards-meeting-oulu/