Vector does reallocation on every push_back

Posted by Amrish on Stack Overflow See other posts from Stack Overflow or by Amrish
Published on 2010-04-27T19:47:31Z Indexed on 2010/04/27 19:53 UTC
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vector

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push-back

IDE - Visual Studio 2008, Visual C++

I have a custom class Class1 with a copy constructor to it.
I also have a vector
Data is inserted using the following code

 Class1* objClass1;
 vector<Class1> vClass1;

 for(int i=0;i<1000;i++) {
objClass1 = new Class1(); vClass1.push_back(*objClass1); delete objClass1; }

Now on every insert, the vector gets re-allocated and all the existing contents are copied to new locations. For example, if the vector has 5 elements and if I insert the 6th one, the previous 5 elements along with the new one gets copied to a new location (I figured it out by adding log statements in the copy constructors.)

On using reserve(), this however does not happen as expected! I have the following questions

Is it mandatory to always use the reserve statement?
Does vector does a reallocation every time I do a push_back; or does it happen because I am debugging?

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