CODEWITHSUNDEEP
c++c++11constexpr
skypjack
skypjack

Reputation: 50548

constexpr and function body = delete: what's the purpose?

According to the [dcl.constexpr/3]:

The definition of a constexpr function shall satisfy the following requirements:
[...]
- its function-body shall be = delete, = default, or [...]

This means that the following class snippet is valid:

struct S {
    constexpr void f() = delete;
};

What's the purpose of having a deleted constexpr function?
What are the benefit of defining it constexpr if any?

I can't figure out any reason, but the fact that maybe it's easier to allow it than to forbid it in the standard.

Upvotes: 2

Views: 409

Answers (2)

Barry
Barry

Reputation: 303537

This was based on CWG 1199. Daniel Krügler wrote:

it could be useful to allow this form in a case where a single piece of code is used in multiple configurations, in some of which the function is constexpr and others deleted; having to update all declarations of the function to remove the constexpr specifier is unnecessarily onerous.

Upvotes: 3

Anedar
Anedar

Reputation: 4275

I guess the purpose is the same as with any =delete:

If you inherit from a class, but don't want the function to be available in subclasses.

E.g.:

class P{
    public:
    constexpr int foo(){return 42;}

};

class C : public P{
    public:
    constexpr int foo() = delete; //(*)
};

int main() {
    P p;
    cout << p.foo(); //ok

    C c;    
    cout << c.foo(); //Compiler error only if line (*) is present.
    return 0;
}

Even though i can't tell you right now where it is usefull - but i also can't see any reason right now why it should be forbidden.

Upvotes: 1

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