C++ Templates & Generic Programming: Function Templates, Class Templates, Specialization & SFINAE (C++23)

How Template Instantiation Works
Function Templates: Type Deduction Rules
Class Templates: Building Generic Containers
Non-Type Template Parameters (NTTPs)
Template Specialization: Full and Partial
Variadic Templates and Fold Expressions
if constexpr: Compile-Time Branching (C++17)
SFINAE and std::enable_if: Constraining Templates
Variable Templates (C++14)
Template Instantiation and Code Bloat
Frequently Asked Questions
Key Takeaway

How Template Instantiation Works

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Templates are not compiled until instantiated. The compiler generates a fresh, type-specific version for each unique set of type arguments. Each version is a fully independent function — no virtual dispatch, no boxing, full inlining.

Function Templates: Type Deduction Rules

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Class Templates: Building Generic Containers

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Non-Type Template Parameters (NTTPs)

NTTPs let you pass compile-time constants as template arguments — enabling truly zero-overhead fixed-size abstractions:

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Template Specialization: Full and Partial

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Variadic Templates and Fold Expressions

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if constexpr: Compile-Time Branching (C++17)

if constexpr evaluates the condition at compile time — the false branch is completely discarded, never compiled:

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SFINAE and std::enable_if: Constraining Templates

SFINAE (Substitution Failure Is Not An Error) — if template substitution fails, the overload is silently removed:

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Frequently Asked Questions

Why must templates be in header files? Because the compiler needs the full template definition to instantiate it. When you include a header, the compiler sees the template definition and can generate the specialized code. If the definition is in a .cpp file, other translation units can't instantiate it. Exception: extern template can suppress re-instantiation across TUs, and C++20 modules eliminate the header requirement entirely.

What is "code bloat" and how do I prevent it? Each unique instantiation generates separate machine code. std::vector<int> and std::vector<double> generate separate versions of all member functions — potentially doubling binary size. Mitigations: (1) Use type-erased base implementations (like std::vector<void*> backing smaller wrappers), (2) Use extern template to instantiate in one TU, (3) Use std::function for callables, (4) C++20 modules reduce repetition.

What is the difference between typename and class in template parameters? None — template<typename T> and template<class T> are completely interchangeable. typename is preferred in modern code for clarity (the parameter doesn't need to be a class). The only exception: inside a template body, typename is required to disambiguate dependent type names: typename T::value_type.

Key Takeaway

C++ templates are the bridge between high-level expressiveness and bare-metal performance. Every STL container, algorithm, and utility is built on templates. When you understand instantiation, specialization, and SFINAE, you gain the ability to write zero-overhead generic libraries that are impossible in any other mainstream language. C++20 Concepts (next module) make templates readable — the final piece of the template mastery puzzle.

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C++ Templates & Generic Programming: Function Templates, Class Templates, Specialization & SFINAE (C++23)