Templates in Modern C++

Overview

Templates enable writing generic, type-independent code in C++. They are the foundation for reusable algorithms and libraries like the STL.

Templates allow you to: - Write a function or class once and reuse it with many types. - Rely on compile-time type deduction for efficiency and safety. - Specialize behavior for certain types when necessary.

Types of Templates

C++ provides several kinds of templates:

1. Function templates — for generic algorithms.

2. Class templates — for type-parameterized data structures.

3. Variable templates — for parameterized constants.

4. Alias templates — for reusable type aliases.

Note

Templates are instantiated at compile time. The compiler generates a concrete version of the template for each unique type combination used.

Quick Comparison

Table 1 Template Categories

Template Type	Example	Typical Use
Function template	template<typename T> T add(T a, T b);	Generic algorithms or utilities.
Class template	template<typename T> class Vector;	Data structures (STL containers, smart pointers).
Variable template	template<typename T> constexpr T pi = T(3.14159);	Type-safe constants.
Alias template	template<typename T> using Ptr = std::shared_ptr<T>;	Simplified type aliases.

Detailed Topics

The following sections explore each area of templates in detail:

Template Topics

• Templates — Overview
  • Benefits
  • Templates vs Overloading
• Template Syntax & Organization
  • Function Template
  • Where to Place Definitions
  • Doxygen Documentation
• Template Specialization
  • Full Specialization
  • Guidance
• Type Deduction & Failures
  • Basic Deduction
  • References and const
  • Explicit Arguments
  • When Deduction Fails

Interoperability with Generic Code

Templates integrate with: - Type traits and concepts (in C++20+) for constraints and static checks. - constexpr and decltype for compile-time computation and type inference.

Example

#include <iostream>

template <typename T>
auto multiply(const T& a, const T& b) -> decltype(a * b) {
    return a * b;
}

int main() {
    std::cout << multiply(3, 5) << '\n';       // int
    std::cout << multiply(2.5, 4.0) << '\n';   // double
}

Summary

• Templates are compile-time generators of code.

• They allow zero-cost abstractions and strong type safety.

• Modern C++ encourages constrained templates using concepts.

• For common generic utilities, prefer STL templates (e.g., std::vector).