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Why does std::any_cast throw a std::bad_any_cast exception when an implicit conversion from the actual stored type to the requested type would be possible?
For example:
std::any a = 10; // holds an int now auto b = std::any_cast<long>(a); // throws bad_any_cast exception Why is this not allowed and is there a workaround to allow an implicit conversion (in case the exact type that std::any holds is unknown)?
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Implicit conversions allow the compiler to treat values of a type as values of another type. There's at least one set of scenarios in which this is unambiguously bad: non-total conversions. That is, converting an A to a B when there exists A s for which this conversion is impossible.
An implicit conversion sequence is the sequence of conversions required to convert an argument in a function call to the type of the corresponding parameter in a function declaration. The compiler tries to determine an implicit conversion sequence for each argument.
std::any_cast is specified in terms of typeid. To quote cppreference on this:
Throws
std::bad_any_castif thetypeidof the requestedValueTypedoes not match that of the contents of operand.
Since typeid doesn't allow the implementation to "figure out" an implicit conversion is possible, there's no way (to my knowledge) that any_cast can know it's possible either.
To put it otherwise, the type erasure provided by std::any relies on information available only at run-time. And that information is not quite as rich as the information the compiler has for figuring out conversions. That's the cost of type erasure in C++17.
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