COMS W4995 C++ Deep Dive for C Programmers

Vec Class Template

Lecture outline

1. turning IntArray into a class template

   • change IntArray to Vec
   • change data type to T
   • add template <typename T>
   • push_back() takes const T& now
   • change operator<<() to a function template
   • need to specify element type when declaring Vec – Vec<int> and Vec<string>

2. Value semantics of STL containers

   • all STL containers (i.e., C++ standard library containers) hold their elements by value

3. Stack & heap diagram for the following sequence of code:

   Vec<MyString> v;
   v.push_back("abc");
   v.push_back("def");
   MyString s{"xyz"};
   v.push_back(s);
   

   • Vec<MyString> holds MyString objects by value

   • There is no copy construction of MyString objects

     • MyString objects are default-constructed when capacity increases
     • push_back() invokes MyString::operator=()

4. Placement new

   • std::vector does not default-construct elements on empty slots
   • It simply allocates memory to hold future objects
   • How does vector::push_back() construct an object at existing memory?

   • Placement new syntax:

       new (p) MyString{"xyz"};
     

5. Strong Exception Guarantee

   • IntArray -> Vec changes
   • std::vector move vs. copy