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[expr.const] p5 neither allocation nor deallocation function is a constexpr function #4947

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xmh0511 opened this issue Sep 28, 2021 · 8 comments
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[expr.const] p5 neither allocation nor deallocation function is a constexpr function #4947

xmh0511 opened this issue Sep 28, 2021 · 8 comments

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@xmh0511
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xmh0511 commented Sep 28, 2021 •
edited 

#include <iostream>
void* operator new(std::size_t n){
    return malloc(n);
}
constexpr int f() {
    auto ptr = new int();  // #1
    delete ptr;  // #2
    return 0;
}
int main() {
    constexpr int v = f();
}

The intent of [expr.const#5.17] and [expr.const#5.18] is to make #1 and #2 be usable in constant expressions. However, regardless of invocation of allocation or deallocation function, they all violate [expr.const#5.2]

An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following:

  • an invocation of a non-constexpr function;

They are contradictory.

Proposal:

an invocation of a non-constexpr function, unless it is a replaceable global allocation/deallocation function invoked by a new-expression or a delete-expression.
@xmh0511 xmh0511 changed the title [expr.const] p5 neither allocation nor deallocation function are constexpr [expr.const] p5 neither allocation nor deallocation function is a constexpr function Sep 28, 2021
@cpplearner
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[expr.new]:

During an evaluation of a constant expression, a call to an allocation function is always omitted.

@jensmaurer
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Nothing to see here, I guess.

@xmh0511
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xmh0511 commented Oct 25, 2021

@cpplearner @jensmaurer Ok, for "allocation function", it is omitted by [expr.new] p14, however, how about "deallocation function"? there is no special rule that omits it during the evaluation of a constant expression.

@xmh0511
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xmh0511 commented Oct 25, 2021 •
edited

@jensmaurer Should we also give a similar special rule in [expr.delete]?

During an evaluation of a constant expression, a call to a deallocation function is always omitted.

@jensmaurer
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Not necessary. If the call to the allocation function is omitted, we also omit the call to the deallocation function for allocations so treated.

@xmh0511
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xmh0511 commented Oct 25, 2021

I wonder which rule specifies that "If the call to the allocation function is omitted, we also omit the call to the deallocation function for allocations"?

@jensmaurer
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[expr.delete] p7 bullets

@xmh0511
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xmh0511 commented Oct 26, 2021

Thanks. I see

If the value of the operand of the delete-expression is not a null pointer value, then:

  • [...]
  • Otherwise, the delete-expression will not call a deallocation function.

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@cpplearner @xmh0511 @jensmaurer