const & constexpr Basics

Some values should never change: a tax rate, the number of squares on a chessboard, pi. Marking them const tells the compiler to reject any accidental reassignment, and constexpr goes further by computing values at compile time. "Const-correctness" — using const everywhere you can — is one of the habits that separates solid C++ from buggy C++. This lesson makes it second nature.

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Think of const as writing a value in permanent ink instead of pencil — you can read it forever, but you can't erase and rewrite it. constexpr is stronger still: it's like a value engraved into the building's cornerstone before the doors even open — fixed at construction time (compile time), so it can be used as part of the structure itself, such as deciding how many rooms (array slots) to build. Both protect you: if you (or a teammate) ever try to change something that was meant to be fixed, the compiler stops you immediately instead of letting a subtle bug slip through.

Rule of thumb: use const by default for anything you don't intend to change; promote to constexpr when the value is a fixed compile-time constant.

1. const: Read-Only Values

Add const before a type and the variable becomes read-only: you must initialize it on the spot, and any later attempt to reassign it is a compile error . This is a feature, not a restriction — it documents intent ("this never changes") and lets the compiler catch accidental writes. By convention, constants are named in UPPER_SNAKE_CASE .

2. constexpr: Compile-Time Constants

constexpr is a promise that a value can be computed at compile time . That unlocks uses where C++ requires a compile-time constant — most notably the size of a fixed array or a . Every constexpr is implicitly const , but it adds the stronger guarantee of compile-time evaluation.

Your turn. Choose the right keyword for each constant — one needs to size an array (so it must be compile-time):

These lines use a constexpr size to build and fill an array. Put them in order:

Open main (C), declare the constexpr size first (B), use it to size the array (A), fill it (D), print data[3] = 3 (E), then return 0; (F) and close (G). N must be a compile-time constant declared before it sizes the array.

3. const References in Parameters

A huge practical use of const is in function parameters. Passing a large object by value copies it; passing by const& hands over a reference (no copy) while promising the function won't modify it. This pattern — const std::string& , const std::vector<T>& — is everywhere in good C++ code: fast and safe.

4. const Member Functions

A member function with const after its parameter list promises not to modify the object it's called on. Getters are the classic example. The payoff: a const object may only call const member functions, so marking your read-only methods const lets them work on constants and documents that they don't mutate state.

A constexpr function is one the compiler can evaluate at compile time when its arguments are themselves constant. The same function still works at run time when given runtime values — you write it once and get both:

Computing values at compile time means zero runtime cost and lets results be used where the language demands a constant. In C++20, consteval forces compile-time evaluation, and constinit guarantees compile-time initialization of globals.

Predict the output (or whether it compiles) before revealing the answer.

No — LIMIT is const , so LIMIT = 10; is an error: "assignment of read-only variable".

In standard C++, int arr[n]; with a non-const n is not allowed (variable-length arrays aren't standard). int brr[m]; with constexpr m is fine — it's a compile-time constant.

Yes — prints 7 — get() is a const member function, so a const object may call it.

Put a constexpr constant and const-correct functions together to compute a circle's area and circumference.

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