Reference and pointer typecasting in C++

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I am able to typecast reference of vector<void*> to reference of vector<Foo*> whereas I am unable to typecast vector<void*> to vector<Foo*>. I am getting the error C2440: 'reinterpret_cast' : cannot convert from 'std::vector<_Ty>' to 'std::vector<_Ty>' why?

And I am able to typecast void* to Foo* without getting compiler error.

C++

void* g =&foo;
reportFooVector2(reinterpret_cast<Foo*>(g));

Below is my entire code.

#include "stdafx.h"
struct Foo
{
  string s;
  int i;
};


void reportFooVector( vector <Foo*> * pvf )
{
 
}


void reportFooVector1( vector <Foo*> pvf )
{
}

void reportFooVector2( Foo *pvf )
{
}


int main()
{
	struct Foo foo = {"foo",  5};
	struct Foo goo = {"goo", 10};
	void* g =&foo;
	reportFooVector2(reinterpret_cast<Foo*>(g));
	vector <void *> vf;
	vf.push_back(&foo);
	vf.push_back(&goo);
	reportFooVector1( reinterpret_cast< vector < Foo * >  >(vf));
	reportFooVector( reinterpret_cast< vector < Foo * > * >(&vf));
}

In the above program I am getting the compiler error C2440: 'reinterpret_cast' : cannot convert from 'std::vector<_Ty>' to 'std::vector<_Ty>' when calling the line reportFooVector1( reinterpret_cast< vector < Foo * > >(vf));

Could you please anyone tell the reason for it?

What I have tried:

I browsed the net but could not get any explanation.

Posted 25-Sep-18 2:18am

steffi123

Updated 26-Sep-18 7:25am

v3

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CPallini · Answer 1 · 2018-09-25T02:30:00

Solution 1

The reason is simple: reinterpret_cast only works with pointers (this is the short answer, for the detailed one, feel free to read the documentation: reinterpret_cast conversion - cppreference.com[^]).

Posted 25-Sep-18 2:30am

CPallini

Updated 25-Sep-18 2:31am

v2

Comments

CPallini 25-Sep-18 8:59am

Exactly.

KarstenK · Answer 2 · 2018-09-25T06:05:00

Type casting can lead to strange errors when done wrong, so it is better to avoid this or handle it with greatest care.

So avoid type casting to void* because the programmer and the compiler are loosing precious informations about the objects. Any up cast is leading to different handling of functions and has so the danger of a crash.

Normally it is a sign of poor design or a coding bug. Maybe NOT in your example, but your real code.

Rick York · Answer 3 · 2018-09-26T07:25:00

One thing I find helpful in these situations is to use type definitions. Here are a couple types I would define in this situation :

C++

typedef struct Foo * pFoo;
typedef std::vector<pFoo>  vecpfoo;

Then I would adjust your functions to this :

C++

void reportFoo( int index, pFoo pf )
{
   cin << "Foo item: " << index << endl;
   cin << "Member s: " << pf->s << endl;
   cin << "Member i: " << pf->i << endl;
}

void reportFooVector( vecpfoo & vpf )
{
   // this should be written with an iterator loop
   int count = (int)vpf.size();
   for( int i; i < count; ++i )
       reportFoo( i+1, vpf[i] );
}

and I would rewrite the main function like this:

C++

int main()
{
   struct Foo foo = { "foo", 5 };
   struct Foo goo = { "goo", 10 };
   vecpfoo vf;
   vf.push_back( &foo );
   vf.push_back( &goo );

   reportFooVector( vf );
}

Reference and pointer typecasting in C++

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