CODEWITHSUNDEEP
c++c++11language-lawyerc++14
Glenn Teitelbaum
Glenn Teitelbaum

Reputation: 10343

C++14: can you call new in a constexpr?

When C++14 lifted restrictions on constexpr it seemed to include the following (copied from Wikipedia):

Expressions may change the value of an object if the lifetime of that object began within the constant expression function. This includes calls to any non-const constexpr-declared non-static member functions.

That seems to imply that you could create an object using new and as long as you delete it within the expression, then it would be allowed.

Upvotes: 7

Views: 2182

Answers (2)

Casey
Casey

Reputation: 42574

Language lawyer answer. All references to N3797.

7.1.5/5 states:

For a non-template, non-defaulted constexpr function or a non-template, non-defaulted, non-inheriting constexpr constructor, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression (5.19), the program is ill-formed; no diagnostic required.

Jumping over to 5.19, we see:

A conditional-expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine (1.9), would evaluate one of the following expressions:

  • ... [lots of bullets]...

  • a new-expression (5.3.4);

  • a delete-expression (5.3.5);

  • ... [lots more bullets]...

So no: a program containing a constexpr function with an unconditional invocation of new or delete in it is ill-formed, no diagnostic required. (I'd be surprised, however, if any half-decent compiler failed to diagnose such invocations of new or delete in a constexpr function, required or not.)

Upvotes: 9

Ferruccio
Ferruccio

Reputation: 100718

I don't think so. You are still limited to calling other constexpr functions. new is actually a function call to operator new() which is not a constexpr function. The same goes for delete.

Upvotes: 2

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