reinterpret_cast<byte*> truncating to 8

I am trying to convert a std::string to cryptopp::byte like this
const byte* pt = reinterpret_cast<const byte*>(&alert[0]);

but it doesn't copy the full string, as confirmed by sizeof
sizeof(alert) = 24
sizeof(pt) = 8

why is it truncating to first 8 bytes?

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