No warning on unused variable

[Andrew Haley]

D Haley writes:

Ah, perhaps a distant cousin.  Hi!

 > I came across this situation recently
 > 
 > Consider the following code
 > 
 > #include <sstream>
 > 
 > using std::stringstream;
 > 
 > int main()
 > {
 >         int i;
 >         stringstream ss;
 > }
 > 
 > I the compiled it with
 > 
 > gcc main.cpp -Wall -lstdc++
 > >main.cpp: In function 'int main()':
 > >main.cpp:7: warning: unused variable 'i'
 > 
 > Why did this not warn me about the unused stringstream?
 > Can wall only catch unused primitive types?

No, it's not that, it's ...

 > Is it because the constructor is called and therefore it is not unused?

Yes.

 > It seems that -Wall should pick this up from the point of view of "i
 > declared it then did nothing, this is incorrect" (also i used no params
 > in the constructor), but it is concievable that declaring this object
 > might be sufficient for some action to be taken by the code, thus
 > rendering it incorrect for -Wall to report it.

You got it in one.  For example, you might see

  {
    lock myLock;
    ...  
  }

where the lock constructor locks memory and the destructor releases
it.  

 > Is this desirable behaviour by GCC or not?

Sure is.

Andrew.