CODEWITHSUNDEEP
c++
Mark
Mark

Reputation: 6464

const reference and creating copy

(Newbie learning c++ with existing codebase) Consider the following snippet:

class Filter
{
    ...
public:
    Filter();
    int addFilter(gFilter &filterDesc);
    …
};

class gFilter 
{
  public:
  ….
  inline const RangeOfUInt&  getProto() const
  {
       return proto; 
  }
  private:
     …
     RangeOfUInt   proto;
     …
};

typedef struct
{
    ValueType  type;
    uint32_t   value1;
    uint32_t   value2;
} UIntRange;

typedef std::list<UIntRange> RangeOfUInt;
…

const RangeOfUInt protos = filterDesc.getProto();   //  XXX

So in this example, getProto() returns const reference and will create a copy for 'protos' ?

Upvotes: 3

Views: 15159

Answers (3)

Alan Stokes
Alan Stokes

Reputation: 18964

Yes, you are exactly right. protos is a copy made from the reference returned from getProto. (The caller creates the copy, getProto doesn't.)

Upvotes: 2

bjackfly
bjackfly

Reputation: 3336

I think if you want a const reference then keep the method as is change and your call as noted by @jayadev to be const RangeOfUInt& protos = filterDesc.getProto(); If you want to modify a copy, then have a method call that just returns a copy. If you are using c++11 most items ( not sure what ValueType is ) seem move constructable and with copy value optimization you will not lose performance it will get constructed directly into the new object. ( see NVRO )

RangeOfUInt protos = filterDesc.getProto();

class gFilter 
{
  public:
  ….
  inline RangeOfUInt  getProto()
  {
       return proto; 
  }
  private:
     …
     RangeOfUInt   proto;
     …
};

Upvotes: 0

jayadev
jayadev

Reputation: 3711

Typically it will create a copy of the protos, 'typically' because compiler might add some optimizations depending on the specific code paths, but while coding we need not worry about that. Since you are looking only for a reference the following change would avoid the copy.

const RangeOfUInt& protos = filterDesc.getProto(); //Make the return variable a reference

Upvotes: 1

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