The order of override and noexcept in the standard

问题

Is the order of override and noexcept required by the standard?

class Base
{
public:
  virtual void foo() {}
};

class Derived : public Base
{
public:
  // virtual void foo() override {} // Ok
  // virtual void foo() noexcept {} // Ok
  // virtual void foo() noexcept override {} // Ok
  virtual void foo() override noexcept {} // Error
};

int main() {}

I'm using gcc 4.7.2.

回答1:

Actually, yes, it is, its just hard to find out, since its a bit scattered. Annex A (Grammar summary) is of some help here. Lets try to find the specific bits:

declarator:
    ptr-declarator
    noptr-declarator parameters-and-qualifiers trailing-return-type

parameters-and-qualifiers:
    ( parameter-declaration-clause ) attribute-specifier-seqopt cv-qualifier-seqopt
    ref-qualifieropt exception-specificationot

exception-specification:
    dynamic-exception-specification
    noexcept-specification

noexcept-specification:
    noexcept ( constant-expression )
    noexcept

and then later for override

member-declarator:
    declarator virt-specifier-seqopt pure-specifieropt
    declarator brace-or-equal-initializeropt
    identifieropt attribute-specifier-seqopt: constant-expression

virt-specifier-seq:
    virt-specifier
    virt-specifier-seq virt-specifier

virt-specifier:
    override
    final

So a declarator is the thing that contains the noexcept keyword, but in the member-declarator the virt-specifier comes after the declarator.

回答2:

In 8.3.5 [dcl.fct] we see:

D1 ( parameter-declaration-clause ) cv-qualifier-seq opt ref-qualifier opt exception-specification opt attribute-specifier-seq opt

...and in 9.2 [class.mem] we see:

declarator virt-specifier-seq opt pure-specifier opt

This states that override and final have to come after noexcept.

回答3:

excerpts from [gram.decl]/A.7, emphasis mine

declarator:
   ptr-declarator
   noptr-declarator parameters-and-qualifiers trailing-return-type

ptr-declarator:
   noptr-declarator
   ptr-operator ptr-declarator

noptr-declarator:
   declarator-id attribute-specifier-seq_opt
   noptr-declarator parameters-and-qualifiers
   noptr-declarator [ constant-expression_opt ] attribute-specifier-seq_opt
   ( ptr-declarator )

parameters-and-qualifiers:
( parameter-declaration-clause ) attribute-specifier-seq_opt cv-qualifier-seq_opt
ref-qualifier_optexception-specification_opt

[gram.class]/A.8, emphasis mine

member-declarator:
   declarator virt-specifier-seq_opt pure-specifier_opt
   declarator brace-or-equal-initializer_opt
   identifier_opt attribute-specifier-seq_opt : constant-expression

virt-specifier-seq:
   virt-specifier
   virt-specifier-seq virt-specifier

virt-specifier:
   override
   final

That is, the exception-specification is part of the declarator, which comes before the virt-specifier in a member-declarator.

来源：https://stackoverflow.com/questions/21577466/the-order-of-override-and-noexcept-in-the-standard