C++ Academy · Lesson

constexpr Functions and if constexpr

Evaluate code at compile time with constexpr and branch with if constexpr.

Lesson 4 of 414 steps

constexpr Functions and if constexpr is a free C++ Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, and your progress syncs across the web and the CoddyKit app. The C++ Academy course includes 4 lessons in total.

Compile-Time Evaluation

The constexpr keyword tells the compiler "evaluate this at compile time when possible." Compile-time computation has zero runtime cost.

constexpr Variables

A constexpr variable must be initialized with a constant expression. The result is fixed at compile time.

constexpr int BUFFER_SIZE = 1024;
constexpr double PI = 3.14159265358979;

constexpr Functions

Mark functions as constexpr to allow them to run at compile time when called with constant arguments.

constexpr int factorial(int n) {
    return (n <= 1) ? 1 : n * factorial(n - 1);
}

constexpr int five_fact = factorial(5);  // computed at compile time

Runtime Fallback

A constexpr function can also be called with runtime values — it runs at runtime in that case.

int n = read_input();
int result = factorial(n);   // runtime call

Restrictions on constexpr Functions

Before C++14:

Only one return statement
No loops
No mutating local state

C++14 relaxed these; C++17 and C++20 added more (try-catch, dynamic alloc, virtuals).

consteval (C++20)

Force compile-time evaluation with consteval. Calling a consteval function with non-constant arguments is an error.

consteval int square(int x) { return x * x; }

constexpr int c = square(5);    // OK
int x = 5;
// int r = square(x);            // ERROR

constinit (C++20)

constinit guarantees the variable is initialized at compile time but allows runtime mutation.

if constexpr (C++17)

if constexpr is a compile-time branch. The non-taken branch is not even compiled — useful for templates.

template <typename T>
void print(const T& value) {
    if constexpr (std::is_pointer_v<T>) {
        std::cout << "pointer: " << *value;
    } else {
        std::cout << value;
    }
}

Eliminating SFINAE

if constexpr replaces complex SFINAE patterns with clear conditional logic. Combined with type traits, it makes generic code far easier to read.

Compile-Time Algorithms

Modern C++ lets you write surprisingly complex algorithms that run entirely at compile time — string hashing, table generation, mini parsers.

constexpr auto fnv1a(std::string_view s) {
    uint32_t h = 2166136261u;
    for (char c : s) {
        h = (h ^ static_cast<uint8_t>(c)) * 16777619u;
    }
    return h;
}

constexpr auto x = fnv1a("hello");

Templates Plus constexpr

Generic constexpr functions are a powerful combination — write generic, type-flexible code that evaluates at compile time.

Diagnostics

Compile-time errors in constexpr code are reported with detailed locations. Modern compilers are quite good at pinpointing the offending step.

Quick Check

What does if constexpr do that a regular if does not?

Answer it in the interactive lesson

Recap

constexpr enables compile-time evaluation; consteval enforces it; constinit guarantees compile-time init. if constexpr branches at compile time — the not-taken branch is never instantiated, replacing SFINAE in many cases.

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Frequently Asked Questions

Is the “constexpr Functions and if constexpr” lesson free?

Yes — any 3 lessons of the C++ Academy learning path, including “constexpr Functions and if constexpr”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The C++ Academy course includes 4 lessons in total.

What will I learn in “constexpr Functions and if constexpr”?

Evaluate code at compile time with constexpr and branch with if constexpr. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C++ Academy?

No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “constexpr Functions and if constexpr” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C++ Academy lesson?

Yes. Every C++ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

Function and Class Templates Revisited
Template Specialization Partial and Full
Variadic Templates and Parameter Packs
constexpr Functions and if constexpr

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