300分一直以来尚未解决的有关问题

300分求一个一直以来尚未解决的问题
C/C++坛子一直以来很少有提问者给300分，我就尝试发一个吧，免得其它版说咱C/C++太寒酸了。不过，不要把300分与下面的问题联系起来，这问题可能连30分也不值，但的确是我尚未彻底弄清楚的疑问。

虽然这贴子发在C++版，但不局限于C++，可以放在C90、C99、C++98、C++2003和C++11等角度下讨论。下面代码在C中的行为很容易弄清楚，因此我着重关心的是C++中的表现。

C/C++ code

struct A{    static int k;    int i;};........int A::k = 10;........A fun( ){ A a; return a; }........fun( ).k = 20;fun( ).i = 30;

fun( ).k是左值，这个已被证明，那么，fun( ).i是左值吗？（提示：不要被赋值运算符的左操作数迷惑了）。

[解决办法]
我个人觉得不是，估计下面的标准描述您也看过。

3.10 Lvalues and rvalues [basic.lval]

2 An lvalue refers to an object or function. Some rvalue expressions—those of class or cv-qualified class type—also refer to objects.47)

47) Expressions such as invocations of constructors and of functions that return a class type refer to objects, and the implementation can invoke a member function upon such objects, but the expressions are not lvalues.
[解决办法]
贴出我的理由：
Some rvalue expressions—those of class or cv-qualified class type—also refer to objects.
关于rvalue expressions 标准有一些附注解释：
Expressions such as invocations of constructors and of functions that return a class type refer to objects, and the implementation can invoke a member function upon such objects, but the expressions are not lvalues.

再配合：
An lvalue for an object is necessary in order to modify the object except that an rvalue of class type can also be used to modify its referent under certain circumstances. [Example: a member function called for an object (9.3) can modify the object. ]

虽然上面是说通过成员函数的调用来修改。
[解决办法]
fun().i 是右值。
参见 n3290 3.10/1 bullet 4，这一其中红色部分。
An rvalue (so called, historically, because rvalues could appear on the right-hand side of an assignment expression) is an xvalue, a temporary object (12.2) or subobject thereof, or a value that is not associated with an object.

下面是一个例子程序证明。

C/C++ code

#include <iostream>struct A{ static int k; int i;};int A::k = 10;A fun( ){ A a; return a; }void f (int& ) { std::cout << "int& " << std::endl; }void f (int&&) { std::cout << "int&&" << std::endl; }int main (){ fun().k = 20; f(fun().i); return 0;}
[解决办法]
探讨

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我的VC 10.0怎么是int&呢?
 

 

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