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That's how dynamic_cast works in conversions from base to derived only if base is polymorphic, or else a compile time error occurs. So, to make base polymorphic, just define it like this:

class base{
virtual void dummy();
};

Also, the whole example has numerous syntax errors. The correct example should look like this:

class base{
virtual void dummy();
};
class derived {};
int main()
{
base* b;
derived* d=dynamic_cast<base*>(b);
}

dynamic_cast needs RTTI and RTTI is available only with polymorphic classes .

dynamic_cast has been designed only for inheritance hierarchy downcasting. Use static_cast or reinterpret_cast. However, reinterpret_cast is not portable across platforms.

class base{ };
class derived : public base { };

int main()
{
    base* b;
    derived d;
    
    b = dynamic_cast<base*>(&d);
    
    return 0;
}