1. CS 2304: Templates And Compilation
Gusukuma 2015, credit to Monti, Allevato 2014

2. Gusukuma 2015, credit to Monti, Allevato 2014
Templates
Syntactically like Java Generics but VASTLY different
However, it’s C++’s version of Generics
Templates can be applied to classes AND functions
At compile time, creates a copy of the class with a different type
E.g. declaring
vector<int> myIntList;
vector<string> mystrList;
Creates TWO vector classes, one for integers and one for strings
Gusukuma 2015, credit to Monti, Allevato 2014

3. Templates: Function Example
// typename can be used instead of class
template <class T>
T sum(const T x, const T y){
return x + y; }
For differences between typename and class, go to stack overflow
Gusukuma 2015, credit to Monti, Allevato 2014

4. Templates: Class Example
Creates a box class that can use doubles or ints
Note that Box<int> and Box<double>creates two COMPLETELY different UNRELATED classes (they don’t even share a base class).
template <class T>
class Box {
public:
T get();
void set(T t);
private:
T elem; };
Gusukuma 2015, credit to Monti, Allevato 2014

5. Templates: Multiple Types
Can have ANY number of types in the parameter list
template <class T, class E>
class Pair {
public:
T get1();
void set1(T t);
E get2();
void set2(E t);
private:
T elem;
E elem2; };
Gusukuma 2015, credit to Monti, Allevato 2014

6. Templates: Parameter Pack (see cppreference.com)
Variable parameters for templates (0 or more parameters)
Functions
Usable for variable arguments
Use static_assert with type casting
Code written LIKE recursion, but not actually recursion
Classes/Structs (see eli.thegreenplace.net)
Usable for data structures of various size
Gusukuma 2015, credit to Monti, Allevato 2014

7. Gusukuma 2015, credit to Monti, Allevato 2014
Templates: Downsides
Can’t use the header/source design pattern
Templates can’t create an address/shortcut, it’s a “template” for code so the code can’t be written until the type is known
Issue of separate compilation units
Easily bloats code
Code generated for each instantiation of a template
Increase compile time
Gusukuma 2015, credit to Monti, Allevato 2014

8. Template Compilation Problem
COPY
Declaration of myFunc<int> exists?
Nope
Template declaration of myFunc exists?
Yup
Creating myFunc<int> from myFunc<T>
Definition of myFunc<int> exists?
Template Definition for myFunc exists?
Creating definition of myFunc<int> from template definition of myFunc<T>
Header
MyHeader
Source1
#include “MyHeader”
myFunc<T>{definition}
myFunc<int>()
Source2
#include “MyHeader”
instantitionOfTemplate<double>
myFunc<T>()
myFunc<T>()
Gusukuma 2015, credit to Monti, Allevato 2014

9. Template Compilation Problem
Existing Code
myFunc<int>{definition}
Template Compilation Problem OK
COPY
Header
MyHeader
Source1
#include “MyHeader”
myFunc<T>{definition}
myFunc<int>()
Source2
#include “MyHeader”
myFunc<int>
myFunc<double>
myFunc<T>()
myFunc<T>()
Gusukuma 2015, credit to Monti, Allevato 2014

10. Template Compilation Problem
Existing Code
myFunc<int>{definition}
Template Compilation Problem OK
Declaration of myFunc<int> exists?
Nope, but definition exists
COPY
COPY
Header
MyHeader
Source1
#include “MyHeader”
myFunc<T>{definition}
myFunc<int>()
Source2
#include “MyHeader”
myFunc<int>
myFunc<double>
myFunc<T>() OK
myFunc<T>()
myFunc<T>()
Gusukuma 2015, credit to Monti, Allevato 2014

11. Template Compilation Problem
Existing Code
myFunc<int>{definition}
Template Compilation Problem OK
COPY
Header
MyHeader
Declaration of myFunc<double> exists?
Nope
Template declaration of myFunc exists?
Yup
Creating myFunc<double> from myFunc<T>
Definition of myFunc<double> exists?
Template Definition for myFunc exists?
Source1
#include “MyHeader”
myFunc<T>{definition}
myFunc<int>()
Source2
#include “MyHeader”
myFunc<int>
myFunc<double>
myFunc<T>()
myFunc<T>() OK
myFunc<T>()
OH NOES
Gusukuma 2015, credit to Monti, Allevato 2014