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Language: C++
Expertise: All
Mar 27, 2000



Building the Right Environment to Support AI, Machine Learning and Deep Learning

Reversing Operands' Order in an Equality Expression

An equality expression takes two operands, the second of which is often a constant or an rvalue:

  if (x==4)  // 4 is an rvalue
  // .. do something

  if (y!=MAX_COUNT) // MAX_COUNT is a constant
  // .. do something

Some programmers prefer to reverse the operands' order:

  if (4==X)
  // .. do something

  if (MAX_COUNT!=y)  
  // .. do something

Because the equality and inequality operators are commutative, reversing the order of their operands doesn't change the result. However, the benefit is that a reversed order saves you from using = instead of == by mistake. If you put = instead of == in a reversed order expression, for example:

if (4=X)  // mistakenly used = instead of ==
  // .. do something

Your compiler will catch the error and complain about an attempt to assign to an rvalue.

Personally, I don't like the reverse order trick because it makes the code somewhat distorted: the programmer's intention is to check the value of x, not the value of the constant 4. Still, if you find yourself mistaking = for ==, you may adopt this coding style. Note that a good compiler should warn about assignments inside conditions, which makes this techniques less needed than it might seem at first.

Danny Kalev
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