Why is there a performance warning on cast pointer to bool?

Question

Extends.

I thought I was being cool when I did something like:

 bool hasParent() {   return this->parentNode ; } 

Even with a (bool) cast, the warning still doesn't go away.

Where this->parentNode is NULL when there is no parent node.

But I'm getting:

 warning C4800: 'Node *' : forcing value to bool 'true' or 'false' (performance warning) 

What's the deal, yo? Why is that a performance warning? I thought it'd be more efficient to not write something like:

  bool hasParent() {   if( this->parentNode )     return true ;   else     return false ; } 

But the second version generates no warnings and the compiler seems a lot happier. Which is faster though?

Answer 1

There's a discussion on Microsoft Connect about this (What is the performance implication of converting to bool in C++?). The example given to Microsoft is:

$ cat -n t.cpp && cl -c -W3 -O2 -nologo -Fa t.cpp 1 bool f1 (int i) 2 { 3 return i & 2; 4 } 5 6 bool f2 (int i) 7 { 8 const bool b = i & 2; 9 return b; 10 } 11 12 bool f3 (int i) 13 { 14 const bool b = 0 != (i & 2); 15 return b; 16 } t.cpp t.cpp(3) : warning C4800: 'int' : forcing value to bool 'true' or 'false' (performance warning) t.cpp(8) : warning C4800: 'int' : forcing value to bool 'true' or 'false' (performance warning) 

And Microsoft's response (from the developer responsible for the warning) is:

This warning is surprisingly helpful, and found a bug in my code just yesterday. I think Martin is taking "performance warning" out of context.

It's not about the generated code, it's about whether or not the programmer has signalled an intent to change a value from int to bool. There is a penalty for that, and the user has the choice to use "int" instead of "bool" consistently (or more likely vice versa) to avoid the "boolifying" codegen. The warning is suppressed in the third case below because he's clearly signalled his intent to accept the int->bool transition.

It is an old warning, and may have outlived its purpose, but it's behaving as designed here

So basically the MS developer seems to be saying that if you want to 'cast' an int to bool you should more properly do it by using "return this->parentNode != 0" instead of an implicit or explicit cast.

Personally, I'd be interested to know more about what kind of bugs the warning uncovers. I'd think that this warning wouldn't have a whole lot of value.

Answer 2

The fact that casting to bool does not make the warning go away is by design:

Casting the expression to type bool will not disable the warning, which is by design.

I would recommend the approach that the MSDN description of warning C4800 recommends:

return this->parentNode != NULL; 

this makes it clear that you are returning true if parentNode is not a null pointer and false if parentNode is a null pointer.