(C++) Exercise #11: Obtaining a std::vector of read-only (smart?) pointers

 

Difficulty: 2/10

Date added: 30th of March 2011

 

This exercise shows that working with smart pointers is not always easy...

 

This exercise is a continuation on Exercise #10: Obtaining a read-only (smart?) pointer.

 

 

 

 

 

The problem

 

Following Exercise #10: Obtaining a read-only (smart?) pointer a programmer has written the following class:

 

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#include <boost/checked_delete.hpp> struct MyStruct {   int m_x;   private:   ~MyStruct() {}   friend void boost::checked_delete<>(MyStruct *); };

 

Writing such a class enables safe forward declarations and forces the user of this class to use smart pointers, which is a good thing [1].

 

This programmer wants to use a class managing a std::vector of boost::shared_ptr of MyStruct, but he/she also wants to let the user obtain a std::vector of read-only smart pointers/pointers, that can be copied freely.

 

The code below shows the choices and some lines that should and should not compile:

 

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#include <vector> #include <boost/checked_delete.hpp> #include <boost/shared_ptr.hpp> struct MyStruct {   int m_x;   private:   ~MyStruct() {}   friend void boost::checked_delete<>(MyStruct *); }; struct MyStructKeeper {   std::vector</* ??? */> Get() const { /* ??? */ }   private:   std::vector<boost::shared_ptr<MyStruct> > m_v; }; int main() {   MyStructKeeper k1;   MyStructKeeper k2;   std::vector</* ??? */> v1 = k1.Get();   const std::vector</* ??? */> v2 = k2.Get();   std::copy(v2.begin(),v2.end(),std::back_inserter(v1));   v1[0]->m_x = 0; //Should not compile   delete v1[0].get(); //Should not compile }

 

What should the question marks be?

 

View the answer of this part of the exercise

 

 

 

 

 

References

 

1. Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. 2005. ISBN: 0-32-111358-6. Chapter 13: 'Ensure resources are owned by objects. Use explicit RAII and smart pointers.