CODEWITHSUNDEEP
c++
brett
brett

Reputation: 5649

Problem with dynamic casting

This code returns me an error whenever I try to run this code. Can some one please help me. 

struct m
{
    virtual int s( )
    { 
        return 1;
    }
};

struct n : public m
{
    int s( )
    { 
        return 2;
    }
};

int o( )
{
    n* p=new m;
    m* q=dynamic_cast<p>;
    return q->s( );
}

Upvotes: 3

Views: 181

Answers (3)

tim
tim

Reputation: 632

Two problems, first you can't allocate an m and call it n in the first line of main. You have a syntatic error in the dynamic_cast. It is dynamic_cast<new type>(some var).

Upvotes: 0

EboMike
EboMike

Reputation: 77722

Next time, please tell us what the error is!

I suppose you're saying that there is a compile error because you're not using dynamic_cast right? You probably meant to say

m* q=dynamic_cast<m *>(p);

In general, dynamic_cast is the devil. Most implementations are insanely slow and might go as far as going string-based class name checks. Unless you really, absolutely need to use dynamic_cast, please use any other method available (such as static_cast).

Upvotes: 2

kennytm
kennytm

Reputation: 523214

These C++ cast operators should be used as

dynamic_cast<newType>(variable)

In your case,

m* q = dynamic_cast<m*>(p);

BTW, are you confusing the role of m and n? n* p = new m is a syntax error because a base class instance cannot be implicitly converted to a derived class instance. In fact, base → derived is the situation where you actually need dynamic_cast, not the other way around (no casting is needed).

Also, consider giving meaningful names to objects.

Upvotes: 7

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