COMP6771 Week 6.2
Function Templates, Class Templates
Polymorphism & Generic Programming
- Polymorphism: Provision of a single interface to entities of different types
- Two types - :
- Static (our focus):
- Function overloading
- Templates (i.e. generic programming)
- std::vector<int>
- std::vector<double>
- Dynamic:
- Related to virtual functions and inheritance - see week 8
- Static (our focus):
-
Genering Programming: Generalising software components to be independent of a particular type
- STL is a great example of generic programming
Function Templates
Without generic programming, to create two logically identical functions that behave in a way that is independent to the type, we have to rely on function overloading.
int min(int a, int b) {
return a < b ? a : b;
}
double min(double a, double b) {
return a < b ? a : b;
}
int main() {
std::cout << min(1, 2) << "\n"; // calls line 1
std::cout << min(1.0, 2.0) << "\n"; // calls line 4
}Explore how this looks in Compiler Explorer
Function Templates
- Function template: Prescription (i.e. instruction) for the compiler to generate particular instances of a function varying by type
- The generation of a templated function for a particular type T only happens when a call to that function is seen during compile time
template <typename T>
T min(T a, T b) {
return a < b ? a : b;
}
int main() {
std::cout << min(1, 2) << "\n"; // calls int min(int, int)
std::cout << min(1.0, 2.0) << "\n"; // calls double min(double, double)
}Explore how this looks in Compiler Explorer
Some Terminology
template type parameter
template <typename T>
T min(T a, T b) {
return a < b ? a : b;
}template parameter list
Type and Nontype Parameters
- Type parameter: Unknown type with no value
- Nontype parameter: Known type with unknown value
#include <iostream>
template <typename T, int size>
T findmin(const std::array<T, size> a) {
T min = a[0];
for (int i = 1; i < size; ++i) {
if (a[i] < min) min = a[i];
}
return min;
}
int main() {
std::array<int, 3> x{ 3, 1, 2 };
std::array<double, 4> y{ 3.3, 1.1, 2.2, 4.4 };
std::cout << "min of x = " << findmin(x) << "\n";
std::cout << "min of x = " << findmin(y) << "\n";
}Compiler deduces T and size from a
Type and Nontype Parameters
- The above example generates the following functions at compile time
- What is "code explosion"? Why do we have to be weary of it?
int findmin(const std::array<int, 3> a) {
int min = a[0];
for (int i = 1; i < 3; ++i) {
if (a[i] < min) min = a[i];
}
return min;
}
double findmin(const std::array<double, 4> a) {
double min = a[0];
for (int i = 1; i < 4; ++i) {
if (a[i] < min) min = a[i];
}
return min;
}Class Templates
- How we would currently make a Stack type
- Issues?
- Administrative nightmare
- Lexical complexity (need to learn all type names)
class IntStack {
public:
void push(int&);
void pop();
int& top();
const int& top() const;
private:
std::vector<int> stack_;
};class DoubleStack {
public:
void push(double&);
void pop();
double& top();
const double& top() const;
private:
std::vector<double> stack_;
};Class Templates
- Creating our first class template
// stack.h
#ifndef STACK_H
#define STACK_H
#include <iostream>
#include <vector>
template <typename T>
class Stack {
public:
friend std::ostream& operator<<(std::ostream& os, const Stack& s) {
for (const auto& i : s.stack_) os << i << " ";
return os;
}
void push(T&);
void pop();
T& top();
const T& top() const;
private:
vector<T> stack_;
};
#endif // STACK_H// stack.h (continued)
template <typename T>
void Stack<T>::push(const T &item) {
stack_.push_back(item);
}
template <typename T>
void Stack<T>::pop() {
stack_.pop_back();
}
template <typename T>
T& Stack<T>::top() {
return stack_.back();
}
template <typename T>
const T& Stack<T>::top() const {
return stack_.back();
}
template <typename T>
bool Stack<T>::empty() const {
return stack_.empty();
}Class Templates
Main function
#include "stack.h"
#include <iostream>
#include <string>
int main() {
Stack<int> s1; // int: template argument
s1.push(1);
s1.push(2);
Stack<int> s2 = s1;
std::cout << s1 << s2 << '\n';
s1.pop();
s1.push(3);
std::cout << s1 << s2 << '\n';
s1.push("hello"); // Fails to compile.
Stack<std::string> string_stack;
string_stack.push("hello");
string_stack.push(1); // Fails to compile.
}Class Templates
template <typename T>
Stack<T>::Stack() { }
template <typename T>
Stack<T>::Stack(const Stack<T> &s) : stack_{s.stack_} { }
template <typename T>
Stack<T>::Stack(Stack<T> &&s) : stack_(std::move(s.stack_)); { }
template <typename T>
Stack<T>& Stack<T>::operator=(const Stack<T> &s) {
stack_ = s.stack_;
}
template <typename T>
Stack<T>& Stack<T>::operator=(Stack<T> &&s) {
stack_ = std::move(s.stack_);
}
template <typename T>
Stack<T>:: ̃Stack() { }Default rule-of-five
default
class Stack {
public:
// Why won't this work if we uncomment the line below?
// Stack(int i) { stack_.push_back(i); }
void push(int&);
void pop();
int& top();
const int& top() const;
private:
std::vector<int> stack_;
};
int main() {
Stack s;
}COMP6771 19T2 - 6.1 - Templates Basics
By cs6771
COMP6771 19T2 - 6.1 - Templates Basics
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