Answer.

In C++, there is compile-time (constancy at compile time) and link-time (at link time) constancy.

• A const variable is an object that cannot be changed after initialization. However, const does not always guarantee that the value is known at compile time; it may only be computed at runtime.
• constexpr ensures that an expression or function will be evaluated at compile time.

Example:

const int x = time(0); // const but NOT constexpr: value is computed at runtime
constexpr int y = 2 + 2; // constexpr: known at compile time

constexpr int square(int x) { return x * x; }
int arr[square(3)]; // array size is a compile-time expression

Use constexpr for constant expressions that need to be available to the compiler, such as for array sizes or template parameters.

Trick question.

Can a function declared as constexpr be called with non-constant arguments?

Answer: Yes! If the arguments are known at compile time, the result will be computed at compile time. If the arguments are only known at runtime, the function will be evaluated like a regular one.

constexpr int double_val(int x) { return x * 2; }

int val = std::rand();
int result = double_val(val); // Will be called at run-time

Examples of real errors due to ignorance of the nuances of the topic.

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Story

One of the modules specified the size of an array using const int, expecting it to be a compile-time constant. On another compiler, this caused an error because the value was computed at run-time, and the array size did not conform to the standard.

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Story

In hash calculations, the compiler could not optimize calculations because a const variable was used instead of constexpr. The result: a performance drop of more than 2 times in new releases.

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Story

When migrating to modern standards, the keywords were confused, and a function was declared as const instead of constexpr, causing the result to be unusable in compile-time expressions of templates. Quick diagnostics revealed the error, but it went into master during code review.