Mastering the C++17 STL

Visiting variants

In the preceding example, we showed how when we had a variable std::variant<int, double> v, calling std::get<double>(v) would give us the current value if the variant currently held a double, but would throw an exception if the variant held an int. This might have struck you as odd--since int is convertible to double, why couldn't it just have given us the converted value?

We can get that behaviour if we want it, but not from std::get. We have to re-express our desire this way: "I have a variant. If it currently holds a double, call it d, then I want to get double(d). If it holds an int i, then I want to get double(i)." That is, we have a list of behaviors in mind, and we want to invoke exactly one of those behaviors on ...