Templates & Generic Programming in C++ (`template<typename T>`)

🔴 Advanced

📖 Definition

Templates are the foundation of Generic Programming in C++. A Template allows you to write a single blueprint function or class that operates on any data type (int, double, std::string, or custom classes) without duplicating code. The compiler generates specialized, type-safe binary code for each used type at compile time (a process called Template Instantiation).

🇮🇳 Hindi Explanation

Templates C++ ka ek powerful feature hai jo aapko generic code likhne ki suvidha deta hai. Agar aapko ek max() function banana hai jo int, double, aur string sab ke liye kaam kare, toh aapko alag-alag 3 functions nahi likhne padenge. Aap template <typename T> ka use karke ek hi generic function likh sakte ho. Compiler automatic type deduce karke executable code bana deta hai.

🚩 Marathi Explanation

Template cha vapar karun aapan C++ madhye type-independent (generic) code lihuu shakto. int, double kiwa string sathi ekach prache function lihinyasathi template <typename T> cha vapar kela jato. Compiler compile-time la lagnaara data type tayar karto.

📝 Function Template & Class Template Syntax

// 1. Function Template Syntax
template <typename T>
T add(T a, T b) {
    return a + b;
}

// 2. Class Template Syntax
template <typename T>
class Box {
private:
    T content;
public:
    Box(T val) : content(val) {}
    T getContent() const { return content; }
};

💡 Practical Example

#include <iostream>
#include <string>

// 1. Generic Function Template
template <typename T>
T findMax(T a, T b) {
    return (a > b) ? a : b;
}

// 2. Generic Class Template (A simple Container Box)
template <typename T>
class StorageBox {
private:
    T item;
public:
    StorageBox(T val) : item(val) {}

    void display() const {
        std::cout << "[StorageBox] Stored Item: " << item << std::endl;
    }
};

int main() {
    std::cout << "--- 1. FUNCTION TEMPLATE DEMO ---" << std::endl;
    std::cout << "Max Int    : " << findMax(10, 25) << std::endl;
    std::cout << "Max Double : " << findMax(88.5, 42.1) << std::endl;
    std::cout << "Max String : " << findMax(std::string("Banana"), std::string("Apple")) << std::endl;

    std::cout << "\n--- 2. CLASS TEMPLATE DEMO ---" << std::endl;
    StorageBox<int> intBox(500);
    StorageBox<double> doubleBox(99.99);
    StorageBox<std::string> stringBox("C++20 Modules");

    intBox.display();
    doubleBox.display();
    stringBox.display();

    return 0;
}

🔍 Code Breakdown

template <typename T>: Tells the compiler that T is a placeholder generic data type.
findMax(10, 25): Compiler automatically deduces T = int and generates an integer version of findMax().
StorageBox<int>: Explicitly instantiates the StorageBox class template with T = int.

👀 Output

--- 1. FUNCTION TEMPLATE DEMO ---
Max Int    : 25
Max Double : 88.5
Max String : Banana

--- 2. CLASS TEMPLATE DEMO ---
[StorageBox] Stored Item: 500
[StorageBox] Stored Item: 99.99
[StorageBox] Stored Item: C++20 Modules

⚠️ Common Mistakes

Separating Template Definitions into .cpp Files: Templates must be fully defined in Header Files (.h / .hpp)! Separating template declarations in header files and implementations in .cpp files leads to undefined reference linker errors.
Incompatible Template Operator Operations: If a template uses a + b, attempting to instantiate the template with a custom class that lacks an overloaded + operator results in a compilation error.

🛡️ Best Practices

Use C++20 Concepts (template <std::integral T>) to constrain template parameters and produce clean, clear compiler error messages when type requirements are violated.

🧪 Try It Yourself

Write a function template swapValues(T& a, T& b) that swaps two variables of any data type.
Create a generic Pair<T1, T2> class template that holds two values of potentially different data types.

🎯 Mini Challenge

Implement a custom generic Stack<T> class template backed by std::vector<T> with push(), pop(), top(), and isEmpty() methods.

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Templates & Generic Programming in C++ (template<typename T>)