Why is there no gcc/g++ warning for unused temporaries?

Question

Consider the following code :

void ListenerImpl::attach(boost::shared_ptr subscriber)
{
    boost::unique_lock(mtx);
          


        

Answer 1

Compiler doesn't issue a warning, because it's quite possible that you might be updating some static-member / global variable inside the constructor (which is valid and meaningful). e.g.:

struct A
{
  static int count;
  A () { count ++; }
};

Now when you simply invoke a temporary:

A();

In case if such update is not happening, compiler will not dig into the constructor of A and check if something useful is happening. It always assumes to be a valid scenario. There are many such cases can be pointed out related to temporaries.