# re: boost::bind綁定成員函數時，第一個參數傳遞對象的特殊情況 2009-06-16 00:14 一個無聊的人

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# re: boost::bind綁定成員函數時，第一個參數傳遞對象的特殊情況 2009-06-16 08:27 董波

6.2. Requirements for Call Wrapper Types
TR1 defines some additional terms that are used to describe requirements for callable types.

First, INVOKE(fn, t1, t2, ..., tN) describes the effect of calling a callable object fn with the arguments t1, t2, ..., tN. Naturally, the effect depends on the type of the callable object. INVOKE is defined as follows:

(t1.*fn)(t2, ..., tN) when fn is a pointer to a member function of a class T and t1 is an object of type T or a reference to an object of type T or a reference to an object of a type derived from T

((*t1).*fn)(t2, ..., tN) when fn is a pointer to a member function of a class T and t1 is not one of the types described in the previous item

t1.*fn when fn is a pointer to member data of a class T and t1 is an object of type T or a reference to an object of type T or a reference to an object of a type derived from T

(*t1).*fn when fn is a pointer to member data of a class T and t1 is not one of the types described in the previous item

fn(t1, t2, ..., tN) in all other cases

What this amounts to is that when the callable object is an ordinary function or a pointer to an ordinary function, INVOKE means to call that function, passing the rest of the arguments to the function call. When the callable object is a pointer to member, the next argument refers to the object that it should be applied to. That argument is the object itself, a reference to the object, a pointer to the object, or some kind of smart pointer that points to the object. The rest of the arguments are passed to the function call.

Second, INVOKE_R(fn, t1, t2, ..., tN, Ret) describes the effect of calling a callable object fn with an explicit return type, Ret. It is defined as INVOKE(fn, t1, t2, ..., tN) implicitly converted to Ret.[5]

[5] In the TR, this metafunction is named INVOKE; although I'm one of the people responsible for this name overloading, I've now concluded that it's too clever and shouldn't be used.

Third, some call wrapper types have a weak result type; this means that they have a nested member named result_type that names a type determined from the call wrapper's target type, Ty.

If Ty is a function, reference to function, pointer to function, or pointer to member function, result_type is a synonym for the return type of Ty

If Ty is a class type with a member type named result_type, result_type is a synonym for Ty::result_type

Otherwise, result_type is not defined[6]

[6] That is, not defined as a consequence of having a weak result type. Some call wrapper types have a weak result type in certain circumstances, have a specific type named result_type

A few examples will help clarify what this rather dense text means:

struct base {
void f();
int g(double);
int h(double,double);
};
struct derived : base {
};

base b;
derived d;
base& br = d;



With these definitions, rule 1 gives the following meanings to these uses of INVOKE .

Phrase
Meaning

INVOKE (&base::f, b)
(b.*f)()

INVOKE (&base::g, d, 1.0)
(d.*f)(1.0)

INVOKE (&base::h, br, 1.0, 2.0)
(br.*f)(1.0, 2.0)





That is, the pointer to member function is called on the object or reference named by t1:

derived *dp = new derived;
base *bp = dp;
shared_ptr<base> sp(bp);



With these additional definitions, rule 2 gives the following meanings to these uses of ( INVOKE):

Phrase
Meaning

INVOKE (&base::f, bp)
((*bp).*f)()

INVOKE (&base::g, dp, 1.0)
((*dp).*f)(1.0)

INVOKE (&base::h, sp, 1.0, 2.0)
((*sp).*f)(1.0, 2.0)





That is, the pointer to member function is called on the object that the argument t1 points to. Since it uniformly dereferences that argument, the rule works for any type whose operator* returns a reference to a suitable object. In particular, the rule works for shared_ptr objects.

Rules 3 and 4 give similar meanings to INVOKE uses that apply pointers to member data:

void func(base&);
struct fun_obj {
void operator()() const;
bool operator()(int) const;
};
fun_obj obj;



With these additional definitions, rule 5 gives the following meanings to these uses of INVOKE:

Phrase
Meaning

INVOKE (func, d)
func(d)

INVOKE (obj)
obj()

INVOKE (obj, 3)
obj(3)


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# re: boost::bind綁定成員函數時，第一個參數傳遞對象的特殊情況 2009-06-16 23:25 wp

文檔上有說明  回復  更多評論   

# re: boost::bind綁定成員函數時，第一個參數傳遞對象的特殊情況 2009-06-19 14:49 釀

哎！我沒耐心看完文檔，有點操之過急了，呵呵，得接受這個教訓  回復  更多評論