Imagine running a grocery shop. You use different delivery boxes for apples, oranges, and eggs etc. But using a separate box for every item leads to complexity. Instead, one general-purpose box that can deliver any item is more efficient. Templates in C++ serve the same purpose: they allow writing a single piece of code that works for multiple data types.

Templates enable generic programming, letting the compiler generate functions or classes for different data types automatically.

What Are Templates?

Template help you write code that can automatically adjust to different data types, making your programs cleaner, shorter, and more efficient.

A template in C++ is a generic blueprint that allows functions or classes
to operate with different data types without rewriting the same code.

In code:

template <typename T>

This introduces T as a placeholder type.

Function Templates

Function templates help avoid writing the same function multiple times for different data types.

Example (Traditional Approach):

int maxInt(int a, int b);
double maxDouble(double a, double b);

Function Template:

template <typename T>
T myMax(T a, T b) {
    return (a > b) ? a : b;
}

The compiler automatically creates versions of this function based on the types used.

Complete Example:

#include <iostream>
using namespace std;

template <typename T>
T add(T a, T b) {
    return a + b;
}

int main() {
    cout << add(3, 4) << endl;
    cout << add(2.5, 3.1) << endl;
    cout << add<string>("Hello ", "World") << endl;
}

Class Templates

Class templates generalize classes to store or operate on values of any type.

Example Without Templates (Repetitive):

class IntBox { int value; };
class FloatBox { float value; };

Class Template:

template <typename T>
class Box {
private:
    T item;
public:
    void set(T value) { item = value; }
    T get() const { return item; }
};

Usage:

Box<int> b1;
b1.set(100);

Box<string> b2;
b2.set("Apples");

Class Template Example

#include <iostream>
using namespace std;

template <class T>
class Calculator {
private:
    T num1, num2;
public:
    Calculator(T a, T b) : num1(a), num2(b) {}
    T add() { return num1 + num2; }
    T subtract() { return num1 - num2; }
};

int main() {
    Calculator<int> obj1(10, 5);
    cout << obj1.add() << endl;

    Calculator<int> obj2(10.5, 2.2);
    cout << obj2.add() << endl;
}

6. Why Templates Matter

• Reusable code: Write once, use for any type.
• Type safety: Compiler handles type correctness.
• Performance: Optimized code generated per type.
• Foundation of STL: The Standard Template Library uses templates extensively.

Introduction to STL (Standard Template Library)

To manage data efficiently in real-world applications, programmers need ready-made structures such as lists, maps, stacks, and sorting functions. Writing these from scratch every time is time‑consuming and error‑prone. The Standard Template Library (STL) provides these tools in a reusable and optimized form.

Here is the student‑friendly definition:

STL (Standard Template Library) is a collection of template-based data structures,
algorithms, and iterators that allow efficient storage, processing, and traversal
of data in C++.

STL provides the following major components:

Containers:

• vector<T>
• list<T>
• queue<T>
• stack<T>
• map<Key,Value>

Algorithms:

• sort()
• find()
• count()

Iterators:

Generic pointers for containers.

STL Example: vector

#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<int> numbers;
    numbers.push_back(10);
    numbers.push_back(20);
    numbers.push_back(30);

    for (int x : numbers)
        cout << x << " ";
}

STL Example: pair<T1,T2>

#include <iostream>
#include <utility>
using namespace std;

int main() {
    pair<string, int> student;
    student.first = "Ali";
    student.second = 22;
    cout << student.first << " is " << student.second << " years old.";
}

STL Algorithm Example: sort()

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    vector<int> data = {4, 1, 9, 2};
    sort(data.begin(), data.end());

    for (int x : data)
        cout << x << " ";
}

Practice Questions

1. Write a function template minValue().
2. Create a class template Storage<T>.
3. Create and print a vector of strings.
4. Sort a vector of doubles.