Is `this` allowed inside a noexcept specification?

问题

I have some code which requires me to use *this, but I want it to be noexcept friendly:

struct foo;

// Would actually be something with conditional noexcept
void do_something(foo&);

struct foo {
    void fn()
        noexcept(noexcept(::do_something(*this)))
    {
        ::do_something(*this);
    }
};

However, gcc rejects this:

<source>:7:43: error: invalid use of 'this' at top level
         noexcept(noexcept(::do_something(*this)))

If I just access a member, gcc is fine:

void do_something(int);

struct bar {
    int x;

    void fn()
        noexcept(noexcept(::do_something(x)))
    {
        ::do_something(x);
    }
};

However, if I access the member through the this pointer, gcc complains again:

struct baz {
    int x;

    void fn()
        noexcept(noexcept(::do_something(this->x)))
    {
        ::do_something(this->x);
    }
};

Diagnostic:

<source>:7:42: error: invalid use of 'this' at top level
         noexcept(noexcept(::do_something(this->x)))

Every other compiler I tried accepts using this inside the noexcept specification, but I don't actually know if it's gcc that has the bug or all the other compilers.

Can the keyword this be used inside a noexcept specification?

回答1:

Yes, it is allowed. [expr.prim.this]p2 says:

If a declaration declares a member function or member function template of a class X, the expression this is a prvalue of type “pointer to cv-qualifier-seq X” between the optional cv-qualifier-seq and the end of the function-definition, [...].

The cv-qualifier-seq refers to the cv qualifiers of the member function, which appear before the noexcept specifier:

parameters-and-qualifiers:
    ( parameter-declaration-clause ) cv-qualifier-seq[opt] ref-qualifier[opt] 
      noexcept-specifier[opt] attribute-specifier-seq[opt]

So, this is a valid expression to use in the noexcept-specifier. This was a DR (cwg1207), which gcc doesn't implement. The bug report.

来源：https://stackoverflow.com/questions/52974329/is-this-allowed-inside-a-noexcept-specification

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language-lawyer

noexcept