[Boost-users] shared_ptr reference conversion and const oddity

From: Brian Hassink (bhassink_at_[hidden])
Date: 2006-12-14 17:09:22

Here's the test case...

 

class A {

  public:

    A() {}

    virtual ~A() {}

};

 

class B : public A {

  public:

    B() {}

};

 

class X1 {

  public:

    X1(const boost::shared_ptr<A>& a) {}

    virtual ~X1() {}

};

 

class Y1 : public X1 {

  public:

    Y1(const boost::shared_ptr<B>& b) : X1(b) {}

};

 

class X2 {

  public:

    X2(boost::shared_ptr<A>& a) {}

    virtual ~X2() {}

};

 

class Y2 : public X2 {

  public:

    Y2(boost::shared_ptr<B>& b) : X2(b) {}

};

 

int main(int argc, char** argv)

{

  boost::shared_ptr<B> b(new B());

 

  Y1 y1(b);  // ptr const in constructor, compiler ok

  Y2 y2(b);  // ptr not const in constructor, compiler not ok

}

 

My question is why conversion of the shared_ptr reference does not work
if it is not declared const?  Here's the compiler error (gcc 3.4.4 -
cygming special)...

 

compiling main.cpp...

main.cpp: In constructor `Y2::Y2(boost::shared_ptr<B>&)':

main.cpp:31: error: no matching function for call to
`X2::X2(boost::shared_ptr<B>&)'

main.cpp:23: note: candidates are: X2::X2(const X2&)

main.cpp:25: note:  X2::X2(boost::shared_ptr<A>&)

make: *** [obj/main.o] Error 1

 

Thanks,

Brian