What's the exact semantics of a deleted functi

问题:

struct A
{
    A();

    A(const A&);
    A& operator =(const A&);

    A(A&&) = delete;
    A& operator =(A&&) = delete;
};

struct B
{
    B();

    B(const B&);
    B& operator =(const B&);    
};

int main()
{
    A a;
    a = A(); // error C2280

    B b;
    b = B(); // OK
}

My compiler is VC++ 2013 RC.

error C2280: 'A &A::operator =(A &&)' : attempting to reference a deleted function

I just wonder why the compiler doesn't try A& operator =(const A&); when A& operator =(A&&) is deleted?

Is this behavior defined by the C++ standard?

回答1:

a = A(); // error C2280

The expression on the right is a temporary which means it will look for operator=(A&&) and sees it is deleted. Hence the error. There is no further search.

=delete does not mean "don't use me, instead use next best one". It rather means, "don't use me when you need me — instead be alone in the wild."

Here is another example. If I want the instances of my class X to be created with only long and no other type (even if it converts into long!), then I would declare class X as:

struct X
{
     X(long arg); //ONLY long - NO int, short, char, double, etc!

     template<typename T>
     X(T) = delete;
};

X a(1);  //error - 1 is int 
X b(1L); //ok    - 1L is long

That means, the overload resolution is performed before the compiler sees the =delete part — and thus results in an error because the selected overload is found deleted.

Hope that helps.

回答2:

When you =delete a function, you actually are deleting its definition.

8.4.3 Deleted definitions [dcl.fct.def.delete]

¹ A function definition of the form:

attribute-specifier-seqopt decl-specifier-seqopt declarator = delete ;

is called a deleted definition. A function with a deleted definition is also called a deleted function.

But by doing so, you are also declaring that function. Quoting from the standard ^[1]:

⁴ A deleted function is implicitly inline. [ Note: The one-definition rule (3.2) applies to deleted definitions. —end note ] A deleted definition of a function shall be the first declaration of the function [...]

And so by doing a = A(), the compiler actually resolves to A::operator=(A&&) because it has been declared (not A::operator(const A&), because A&& is "more binding" to r-values). However with its definition being deleted, the line is ill-formed.

² A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed.

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^[1] The tone of the emphasized sentence here is actually imperative. The standard directs that declaring a function =deleted must first appear before other declarations of it. But still, it supports the fact that deleting a function also declares the function.