What's the reasoning behind putting constants in if statements first?

Question

I was looking at some example C++ code for a hardware interface I\'m working with and noticed a lot of statements along the following lines:

if ( NULL == pMsg )          


        

Answer 1

To clarify what I wrote in some of the comments, here is a reason not to do this in C++ code.

Someone writes, say, a string class and decides to add a cast operator to const char*:

class BadString
{
public:
   BadString(const char* s) : mStr(s) { }

   operator const char*() const { return mStr.c_str(); }

   bool operator==(const BadString& s) { return mStr == s.mStr; }

   // Other stuff...

private:
   std::string mStr;
};

Now someone blindly applies the constant == variable "defensive" programming pattern:

BadString s("foo");

if ("foo" == s) // Oops.  This compares pointers and is never true.
{
   // ...
}

This is, IMO, a more insidious problem than accidental assignment because you can't tell from the call site that anything is obviously wrong.

Of course, the real lessons are:

1. Don't write your own string classes.
2. Avoid implicit cast operators, especially when doing (1).

But sometimes you're dealing with third-party APIs you can't control. For example, the _bstr_t string class common in Windows COM programming suffers from this flaw.