CODEWITHSUNDEEP
c++constexpr
Adam Sperry
Adam Sperry

Reputation: 283

Are there benefits to using a constexpr variable that is never intended to be used in a constant expression?

This is a follow up question to this one.

From one of the answers in the question linked, it makes sense that the purpose of declaring a variable as constexpr is not truly to inform the compiler of the possibility of using that variable as a constant expression (although this is the end behavior of the compiler) but rather to convey to other programmers the intent that the variable can be used in a constant expression. My question is whether there is any benefit, performance or otherwise, to declaring a variable as constexpr if the variable is not intended to be used in a constant expression (e.g. a constant member variable of a class which only appears in non-constant expressions with other non-const member variables). Also assume that this is not for an embedded system.

I work with code where the previous developers seem to use constexpr like it's candy even when it's not clear that the intent is for the variable to be used in a constant expression (like in the example from the previous paragraph). I want to know if they knew something that I don't.

Upvotes: 1

Views: 292

Answers (2)

Maxpm
Maxpm

Reputation: 25602

Yes, constexpr variables are useful for more than just conveying intent and using in other constant expressions.

If you mark a variable constexpr, the compiler will enforce that it itself is initialized with a constant expression. You can use this to make sure that the things you expect to be evaluated at compile time actually are. For example:

constexpr int constexpr_function() { return 123; }
int expensive_runtime_function() { return 123; }

void f()
{
    constexpr int good1 = constexpr_function(); // OK.
    // constexpr int good2 = expensive_runtime_function(); // Good: error.

    const int bad1 = constexpr_function(); // OK.
    const int bad2 = expensive_runtime_function(); // Bad: unexpected runtime cost.
}

Upvotes: 0

R Sahu
R Sahu

Reputation: 206717

but rather to convey to other programmers the intent that the variable can be used in a constant expression.

This is a matter of opinion but I beg to differ.

In principle, I think it is better to make everything that can be known and computed at compile time a constexpr. The fact that it can also be used in constant expressions is a side benefit. Use of constexpr may have performance related benefits too.

The key point I want to make is that "the value is known at compile time" is a characteristic of the variable/data. It is better to capture that characteristic by using a language provided feature. The only time one could justify not using it will be if there are any adverse side effects.

Upvotes: 1

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