Saturday, January 31, 2009


I've always wondered what lvalue and rvalue really meant - and why they were different from 'left operand' and 'right operand'.  It's probably because I know more Java than C++, and Java has lvalue and rvalue, but doesn't allow you to do silly things like '++++i;'.

rvalue: static, read only value, generally a number.  Example: 2,  (2+2), (i+2), i++. Similar to 'const' in C++.
lvalue: Can be assigned to but also has a value, generally a variable or an expression that returns a variable.  Examples:  i, (++i), (i += 3), etc.

The distinction comes in when  you look at the type of a value in an expression, for example:
a = 2 + 3;

In this example, a is an lvalue, 2 and 3 are rvalues, and the sum 5 is also a (hidden) rvalue.

a = ++i;

Here, a is an lvalue because it's being assigned to, and i is an lvalue because it is a variable, but critically, (++i) is also an lvalue because the operator '++' returns the variable it's operating on after the incrementation operation, rather than just the resulting value.  This allows expressions like ++++i to compile and have meaning, because they're effectively (++(++i)).

This begs the question: what does a = (++i = i) do?  No idea. 

Why is this more important to C++ programmers than C programmers?  C programmers have to deal with this rvalue/lvalue distinction only if they're writing expressions that have multiple assignments in them, which you generally shouldn't do because it's confusing to read and understand.   C++ programmers have the same problems, but they also have to understand the rules when they use an advanced C++ feature called operator overloading.  This allows objects to respond to operators like '++' with behaviors specific to that object.  For example, an iterator could move to the next object in the list when ++ is called.  

However, there's a big difference between the preincrement form '++i' and the postincrement form 'i++'.  Preincrement returns the lvalue variable i after it performs the 'increment' operation, and postincrement returns the lvalue variable i before it performs the operation. When a C++ programmer implements postincrement, they have to copy the object that is being incremented, and return the old version after incrementing the new one.


  1. Casinos and games at the Wynn Casino – Del Cidade Gaming
    › › 목포 출장샵 Casino › › Casino Sep 11, 2020 광명 출장마사지 — Sep 11, 2020 Casinos and Games at 밀양 출장안마 the Wynn Casino – Del Cidade Gaming. A high-roller table game, featuring 6-6 제주도 출장안마 players, this is a variation 청주 출장샵 of one of the most popular

  2. The actual money model includes deposit limitations, making it difficult for high-rollers to fulfill their gaming demands. However, many states have already won back their right to both land-based games and on-line casinos. In addition to traditional pc games, games have additionally moved to slot machines. They took game characters and made them symbols that appeal to video games fans. 3D slots — identical as video machines but usually possess more superior 코인카지노 gaming features designed to provide display graphics that give a 3D appeal without a a|with no} need for sporting special glasses. 3D pokies designers consideration to|take note of} images and animations.