C++: const vs constexpr

C++ const vs constexpr: What’s the Real Difference?

Both are used in C++ to define constants.

Why This Matters

Modern C++ encourages writing immutable, efficient, and expressive code. Two keywords—const and constexpr—sit at the heart of that effort. They look similar, but understanding their distinct guarantees is crucial for clean compile‑time and run‑time behavior.

High‑Level Comparison

Feature	const	constexpr
Compile‑time constant?	Maybe	Always (or fails to compile)
Runtime OK?	Yes	Yes (evaluated at runtime if needed)
Array/template usage	Only if truly constant	Guaranteed
Functions allowed?	Only qualifiers on member	Full functions evaluable at CT

1  Declaring Immutable Data

const: Immutable After Construction

const int runtimeConst = std::rand(); // const but NOT a compile‑time constant

When you merely want to prohibit modification—regardless of when the value is known—const is enough.

constexpr: Must Be Known at Compile Time

constexpr int arraySize = 10;
int arr[arraySize];           // Always valid

If the value must participate in contexts that require a constant expression (array bounds, template parameters, switch labels), choose constexpr.

2  Functions and Methods

const Member Functions

class Widget {
public:
    int value() const {/*…*/} // promises not to modify *this
};

They protect object state but offer no compile‑time guarantee.

constexpr Functions

constexpr int square(int n) { return n * n; }
static_assert(square(4) == 16, "computed at compile time");

A constexpr function can run during compilation whenever its arguments are constant expressions, yet it still works at run time.

3  Common Pitfalls

// 1. Compiles: runtimeConst is merely const
const int runtimeConst = std::rand();
// 2. Fails: std::rand() is not constexpr

constexpr int fails = std::rand();

Remember: every constexpr variable is implicitly const, but not every const variable is a constant expression.

4  Guidelines for Choosing

• Need guaranteed compile‑time evaluation? Use constexpr.
• Need immutability but value might come from a run‑time calculation? Use const.
• Prefer constexpr if unsure; the compiler will complain if the initializer isn’t a constant expression.

5  Summary Snippet

constexpr int ctVal = 42; // compile‑time
const int rtVal = std::rand(); // run‑time, still immutable

Choosing correctly between const and constexpr unlocks safer, faster, and more expressive C++ code. Default to constexpr for constants you truly want at compile time, and reserve const for values determined during execution.

C/C++ Programming

• const vs constexpr in C++
• Tutorial on C++ Ranges
• Tutorial on C++ Smart Pointers
• Tutorial on C++ Future, Async and Promise
• The Memory Manager in C/C++: Heap vs Stack
• The String Memory Comparision Function memcmp() in C/C++
• Modern C++ Language Features
• Comparisions of push_back() and emplace_back() in C++ std::vector
• C++ Coding Reference: is_sorted_until() and is_sorted()
• C++ Coding Reference: iota() Setting Incrementing Values to Arrays or Vectors
• C++ Coding Reference: next_permutation() and prev_permutation()
• C++ Coding Reference: count() and count_if()
• C++ Code Reference: std::accumulate() and Examples
• C++ Coding Reference: sort() and stable_sort()
• The Next Permutation Algorithm in C++ std::next_permutation()

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