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下面这段引文可对左值、右值加深理解。

参考文献:《ANSI/ISO C++ Professional Programmer's Handbook》

lvalues and rvalues
An object is a contiguous region of storage. An lvalue is an expression that refers to such an object. The original
definition of lvalue referred to an object that can appear on the left-hand side of an assignment. However, const
objects are lvalues that cannot be used in the left-hand side of an assignment. Similarly, an expression that can appear
in the right-hand side of an expression (but not in the left-hand side of an expression) is an rvalue. For example

#include  < string >
using   namespace  std;
int &  f();
void  func()
{
    
int  n;
    
char  buf[ 3 ];
    n 
=   5 //  n is an lvalue; 5 is an rvalue
    buf[ 0 =   ' a ' //  buf[0] is an lvalue, 'a' is an rvalue
     string  s1  =   " a " , s2  =   " b " , s3  =    " c " //  "a", "b", "c" are rvalues
    s1  =   //  lvalue
    s2  + s3;  // s2 and s3 are lvalues that are  implicitly converted to rvalues
    s1  =   // lvalue
     string ( " z " );  // temporaries are rvalues
     int   *  p  =   new   int // p is an lvalue; 'new int' is an rvalue
    f()  =   0 // a function call that returns a reference is an lvalue
     s1.size();  // otherwise, a function call is an rvalue expression
}

An lvalue can appear in a context that requires an rvalue; in this case, the lvalue is implicitly converted to an rvalue.
An rvalue cannot be converted to an lvalue. Therefore, it is possible to use every lvalue expression in the example as
an rvalue, but not vice versa.
posted on 2006-06-02 12:03 TH 阅读(423) 评论(0)  编辑 收藏 引用

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