<<cpp primer plus 6th edition>>中的原文（Chapter 8 Adventures in Functions）：
What is an lvalue? An argument that’s an lvalue is a data object that can be referenced
by address. For example, a variable, an array element, a structure member, a reference, and
a dereferenced pointer are lvalues. Non-lvalues include literal constants (aside from quoted
strings, which are represented by their addresses) and expressions with multiple terms.The
term lvalue in C originally meant entities that could appear on the left side of an assignment
statement, but that was before the const keyword was introduced. Now both a regular
variable and a const variable would be considered lvalues because both can be
accessed by address. But the regular variable can be further characterized as being a
modifiable lvalue and the const variable as a non-modifiable lvalue.
在第五版对应章节中中作者解释的比较简单，第六版进行了更详细的解释。第五版原文如下：
An argument that’s an lvalue is a data object that can be referenced. For example, a variable,
an array element, a structure member, a reference, and a dereferenced pointer are lvalues.
Non-lvalues include literal constants and expressions with multiple terms.
第五版已有中文版，第六版中文版貌似亚马逊上本月（2013.07月）已上架。
另外c++11还增加了右值(rvalue)的概念,声明方式为:int && num=10;使用&&来表示。