Treat vector<int*> as vector<const int*> without copying (C++0x)

问题

A class contains a std::vector<int*>. External code needs read-only access to this vector, should not be able to modify the contents (neither the pointers or their contents). Inside the class, the values may change (e.g. double_values(), and so storing them as a std::vector<const int*> is not possible.

Is there a way to return the std::vector<int*> as a std::vector<const int*> without making a copy? It feels like there should be, because const is simply operating at compile time to say what can and cannot be modified.

Code: (compile with g++ -std=c++0x)

class ReadOnlyAccess
{
public:
  ReadOnlyAccess(const std::vector<int*> & int_ptrs_param):
    int_ptrs(int_ptrs_param)
  {
  }
  const std::vector<int*> & get_int_ptrs() const
  {
    return int_ptrs;
  }
  std::vector<const int*> safely_get_int_ptrs() const
  {
    // will not compile (too bad):
    //    return int_ptrs;

    // need to copy entire vector
    std::vector<const int*> result(int_ptrs.size());
    for (int k=0; k<int_ptrs.size(); k++)
      result[k] = int_ptrs[k];
    return result;
  }
  void double_values()
  {
    for (int*p : int_ptrs)
      *p *= 2;
  }
  void print() const
  {
    for (const int * p : int_ptrs)
      std::cout << *p << " ";
    std::cout << std::endl;
  }
private:
  std::vector<int*> int_ptrs;
};

int main() {
  ReadOnlyAccess roa(std::vector<int*>{new int(10), new int(20), new int(100)});
  std::vector<const int*> safe_int_ptrs = roa.safely_get_int_ptrs();
  // does not compile (good)
  // *safe_int_ptrs[0] = -100000;
  roa.print();

  const std::vector<int*> & int_ptrs = roa.get_int_ptrs();
  // changes are made to the internal class values via the accessor! nooooo!
  *int_ptrs[0] = -100000;
  roa.print();

  return 0;
}

回答1:

Returning the vector will imply a copy if you want to keep the const pointers anyway.

However, if your goal is to provide a way to use the values without modifying them, or modifying it's container, then a visitor pattern based algorithm might be a very good solution, in particular now that we can use lambda expressions:

#include <vector>
#include <iostream>

class Data
{
public:

    //...whatever needed to fill the values

    // here we assume that Func is equivalent to std::function< void ( int )> or std::function< void (const int& ) > and can return anything that will be ignored here.
    template< class Func > 
    void for_each_value( Func func ) const // read-only
    {
        for( const int* value : m_values ) // implicit conversion
        {
             func( *value ); // read-only reference (const &), or copy
             // if func needs to work with the adress of the object, it still can by getting a reference to it and using & to get it's adress
        }
    }


    void print() const
    {
        std::cout << "\nData values: \n";
        for_each_value( []( const int value ) { std::cout << "    "<< value << '\n'; } );
    }

    void count_values() const { return m_values.size(); }

private:

    std::vector<int*> m_values;

};



int main()
{
    Data data;
    // ... whatever needed to fill the data

    data.print();    

    std::vector<int> modified_values;
    data.for_each_value( [&]( int value ) { modified_values.push_back( value + 42 ); } );

    return 0;
}

If you understand that, and the different ways to use the values can be reduced to a few half-generic algorithms, then it will make your code simpler and allow you to keep data inside your structures instead of exposing it's the guts.

回答2:

You can provide a view to const values via custom iterators. An easy way would be to use boost::iterator:

#include <boost/iterator/indirect_iterator.hpp>

class ReadOnlyAccess
{
// ...
    typedef boost::indirect_iterator<const int* const*, const int> const_val_iter_type;
    const_val_iter_type cval_begin() {
        return it_t{const_cast<const int* const*>(&int_ptrs[0])};
    }
}

int main() {
    // ...
    auto x = roa.cval_begin();
    std::cout << x[0] <<' ' << x[1] << x[2] <<'\n';
    // we can still access the pointers themselves via .base() member function:
    for (int i=0; i<3; ++i)
        assert(x.base()[i] == safe_int_ptrs[i]);
    // the values are read-only, the following does not compile:
    // x[0] = -1;
    // **x.base() = -1;
    // *x.base() = nullptr;
}

If we used boost::indirect_iterator<typename std::vector<int*>::const_iterator, const int> for const_val_iter_type, we could modify the pointed values via .base() (but not directly like in e.g. x[0] = -1), so this solution is not general.

来源：https://stackoverflow.com/questions/10265695/treat-vectorint-as-vectorconst-int-without-copying-c0x