at runtime how to determine actual type of non-polymorphic type

Question

I have such declarations:

struct InstrumentInfo
{
    std::string Name;
    TradingStatus Status;
    myDecimal MinStep;
    std::string ISIN;
    myDecimal limit_down;
    myDecimal limit_up;
};

struct FortsDerivativeInfo : InstrumentInfo
{
    std::string ShortIsin;
    int IsinId;
    std::string CodeVcb;
    myDecimal StepPrice;
    int LotVolume;
    myDecimal exch_pay;
};

struct StockInfo : InstrumentInfo
{
    int LotSize;
};

I've trited to write such code:

if (auto si = dynamic_cast<StockInfo*>(ii))
{
    LOT_SIZE = si->LotSize;
}
else
{
    LOT_SIZE = 1;
}

This doesn't compile, I receive "error C2683: 'dynamic_cast' : 'InstrumentInfo' is not a polymorphic type". How can I fix this error?

If I replace dynamic_cast to static_cast code compiles, but as static_cast doesn't perform any runtime checks I'm afraid this will not work?

possible duplicate of [Regular cast vs. static\_cast vs. dynamic\_cast](http://stackoverflow.com/questions/28002/regular-cast-vs-static-cast-vs-dynamic-cast) — danielschemmel, May 07 '14 at 14:52
@gha.st: I would not say so. The issue is less about choosing the appropriate cast and more about making `dynamic_cast` work or coding an alternative. — Matthieu M., May 07 '14 at 15:10

score 2 · Accepted Answer · answered May 07 '14 at 14:52

Run-time type information is available only for types that have at least one virtual function (i.e. "polymorphic types"). The only way to make dynamic_cast work is to make the base type polymorphic - for example, by declaring its destructor virtual.

A better approach would be removing the need for the cast altogether - for example, by making a virtual lotSize function:

struct InstrumentInfo
{
    std::string Name;
    TradingStatus Status;
    myDecimal MinStep;
    std::string ISIN;
    myDecimal limit_down;
    myDecimal limit_up;
    virtual int lotSize() const { return 1; }
};

struct StockInfo : InstrumentInfo
{
    int LotSize;
    virtual int lotSize() const { return LotSize; }
};

InstrumentInfo ii;
LOT_SIZE = ii.lotSize();

score 0 · Answer 2 · answered May 07 '14 at 15:04

No run-time type information is available for non-polymorphic types (types without virtual functions).

Adding a virtual destructor in InstrumentInfo will give you RTTI and make dynamic_cast work, though the better way to solve your problem is, as pointed out in the other answer, to use a virtual member function that returns lot size. It's a good idea to use a virtual destructor regardless of RTTI anyway, because otherwise you have some serious problems if you do this:

InstrumentInfo * info = new FortsDerivativeInfo;
delete info;

This results in undefined behavior (deleting a derived object through a base class pointer without a virtual destructor).

score 0 · Answer 3 · answered May 07 '14 at 15:06

The short answer is: dynamic_cast only works on polymorphic types, so you have to look for alternatives.

The longer answer is that you have alternatives:

the simplest is to add a virtual method (the destructor of the base class), and your code will compile as is
but in general, if you can use virtual method, create one that will avoid this ugly cast
other alternatives exist, such as having an enum in the base class that indicate which child is currently instantiated; they are not as automated so are of last recourse

at runtime how to determine actual type of non-polymorphic type

3 Answers3