What are the differences between std::variant and boost::variant?

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  • Assignment/emplacement behavior:

    • boost::variant may allocate memory when performing assignment into a live variant. There are a number of rules that govern when this can happen, so whether a boost::variant will allocate memory depends on the Ts it is instantiated with.

    • std::variant will never dynamically allocate memory. However, as a concession to the complex rules of C++ objects, if an assignment/emplacement throws, then the variant may enter the “valueless_by_exception” state. In this state, the variant cannot be visited, nor will any of the other functions for accessing a specific member work.

      You can only enter this state if assignment/emplacement throws.

  • Boost.Variant includes recursive_variant, which allows a variant to contain itself. They’re essentially special wrappers around a pointer to a boost::variant, but they are tied into the visitation machinery.

    std::variant has no such helper type.

  • std::variant offers more use of post-C++11 features. For example:

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