Best way to replace a set in C++

问题:

I have to refactor old code that looks like this (I am not a very proficient C++ coder)

 std::set<SomeObject>::iterator it = setobject.begin();

 do {
     it->setProperty1ToNextValue();
     it->getProperty2();
     it->getProperty3();
     it++ 
 } while (it != setobject.end());

Basically I want to iterate through the elements of the set and get and set/update some of their properties. I cant use the original set since I run in the problems described in this thread the object has type qualifiers that are not compatible with the member function object type is const

I am thinking of replacing the set with a dequeue (it will involve some rewriting) since I will then be able to set and get properties on each element of the dequeue. Is this a good approach?

回答1:

A std::set works by keeping all the items in order according to the object's < operator. The reason you cannot call non-const methods on your object is because there is risk that those methods will change the value returned by your objects < operator, and therefore effectively "reorder" the set under the hood without the std::set knowing.

While you haven't specified enough of what you are trying to do for us to provide the best answer, here is a couple ways to technically achieve calling some methods on your set. You can use const_cast to call methods you are sure won't modify the key. Or you can put the items in a vector, call methods that might modify the "key", and then put them back in the original set.

// Example program
#include <iostream>
#include <string>
#include <set>
#include <algorithm>

class SomeObject
{
    std::string key;
    int         data;

public:
    SomeObject( const std::string& key_, int data_ ) : key( key_ ), data( data_ )
    {}

    // For a item to be in a set, it must have a "<" operator that tells it how to order it
    bool operator <( const SomeObject& rhs ) const
    {
        return key < rhs.key;   
    }

    void setKey( const std::string& key_ )
    {
        key = key_;   
    }

    void setData( int data_ )
    {
        data = data_;   
    }
};

int main()
{   
     std::set< SomeObject > setobject;
     setobject.insert( SomeObject("c", 1 ) );
     setobject.insert( SomeObject("a", 1 ) );
     setobject.insert( SomeObject("b", 1 ) );

     // internally, the set will keep everything in order "a", "b", "c"

     // option 1 - use const_cast (risky!)
     {
         std::set< SomeObject >::iterator it = setobject.begin();

         do {
             // const_cast< SomeObject& >( *it ).setKey( "d" );  bad idea, now the set is jacked up because its not in the right order
             const_cast< SomeObject& >( *it ).setData( 2 );
             it++;
         } while (it != setobject.end());
     }

     // option 2 - put the items in the vector, call the methods, then put them back in the original set
     {
         std::vector< SomeObject > tempVec( std::begin( setobject ), std::end( setobject ) );
         std::vector< SomeObject >::iterator it = tempVec.begin();
         do {
             it->setKey( "d" ); 
             it->setData( 2 );
             it++;
         } while (it != tempVec.end());

         std::set< SomeObject > newSet( std::begin( tempVec ), std::end( tempVec ) );
         std::swap( newSet, setobject );  // put the new items back in the original setobject
     }



}