Avoid multiple copying in vector in C++ - c++

Question

My question is give below to avoid multiple copies in vector copying.
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
class DataValue {
public:
DataValue() { std::cout << "DataValue constructor called" << std::endl; }
DataValue(DataValue const& other) { cout << "DataValue copy constructor called" << std::endl; }
~DataValue() { std::cout << "DataValue destructor is called" << std::endl; }
private:
};
class ItemDataHistory {
public:
ItemDataHistory() { std::cout << "ItemDataHistory constructor called" << std::endl; }
// ItemDataHistory(ItemDataHistory const & other) { std::cout << "ItemDataHistory copy constructor called" << std::endl; }
~ItemDataHistory() { std::cout << "ItemDataHistory destructor called" << std::endl; }
std::vector<DataValue>& GetVecDataValues() { return m_vecDataValues; }
private:
std::vector<DataValue> m_vecDataValues;
};
class DataReply {
public:
DataReply() { std::cout << "Data reply constructor is called "<< std::endl; }
~DataReply() { std::cout << "Data reply destructor is called "<< std::endl; }
DataReply(const DataReply& ) { std::cout << "Data reply copy constructor is called "<< std::endl; }
std::vector<ItemDataHistory>& GetItemDataHistories() { return m_vecItemData; }
private:
// The list of DataValue
std::vector<ItemDataHistory> m_vecItemData;
};
void main()
{
DataValue dv1, dv2, dv3;
ItemDataHistory itmDH;
itmDH.GetVecDataValues().reserve(3);
itmDH.GetVecDataValues().push_back(dv1);
itmDH.GetVecDataValues().push_back(dv2);
itmDH.GetVecDataValues().push_back(dv3);
DataReply dr;
dr.GetItemDataHistories().reserve(1);
dr.GetItemDataHistories().push_back(itmDH); // Here copy consturtor of itemdatahistory is called and all data values are copied.
// Here I want to avoid data values constructor to be called again how can I avoid this
// How can I directly insert values of dv1, dv2, dv3 into "dr" with out using "itmDH"?
return;
}
Note here I cannot use pointer in above std::vector m_vecItemData; in data reply class as these are interface classes from libary and don't have control on it and I am calling function so function may use data while data in scope
My question is given in above comment in code. Reason is that I have thousands of data values. To avoid multiple constructors of data values to be called, I want to insert data values directly to data reply (i.e., with out using itmDH local variable)
and other questions is
How I can reserve space of data values inside data reply?

Answer 1

With C++11, you have two options:
make your type ItemDataHistory movable and move your data (if possible) with dr.GetItemDataHistories().push_back(std::move(itmDH));
look into new member function of containers, e.g. emplace_back().

Answer 2

In C++11, you can use move semantics.
Instead of doing this:
itmDH.GetVecDataValues().push_back(dv1);
itmDH.GetVecDataValues().push_back(dv2);
itmDH.GetVecDataValues().push_back(dv3);
You could do this:
itmDH.GetVecDataValues().push_back(std::move(dv1));
itmDH.GetVecDataValues().push_back(std::move(dv2));
itmDH.GetVecDataValues().push_back(std::move(dv3));
Instead of copying values, they are simply moved into the vector.
And instead of copying itmDH
dr.GetItemDataHistories().push_back(itmDH);
you could move it as well:
dr.GetItemDataHistories().push_back(std::move(itmDH));
In addition you also need move constructors. Here's an example:
DataValue(DataValue&& other){
std::cout << "DataValue move constructor called" << std::endl;
}
You may also declare and define move assignment operator:
DataValue& operator=(DataValue&& other){
std::cout << "DataValue move assigment operator is called" << std::endl;
return *this;
}
In order to fully understand move semantics (and rvalue references as well) please take a look at the following links:
http://www.cprogramming.com/c++11/rvalue-references-and-move-semantics-in-c++11.html
http://thbecker.net/articles/rvalue_references/section_01.html