Is it incorrect to modify a temporary?

Question

In C++98/03/11 the following code is valid:

std::string meow() { return "meow"; }

int main()
{
    meow().append("purr");
    return 0;
}

Since the temporary dies at the semi-colon, this should be safe right?

Answer 1

It is not incorrect and it is useful to do this in certain circumstances.

Suppose I have a vector with a lot of data and I want to clear the data and the memory..

{ 
   std::vector< int >().swap( myVec );
}

will clear the memory for certain. myVec.clear() will probably only change the logical size back to 0.

As I showed in a previous question you can use it like this:

class Foo
{
   public:
       Foo& operator+=( Foo const& ); // implement
       Foo operator+( Foo const& rhs ) const
       {
          return Foo( *this ) += rhs;
       }
 };

The implementation there modifies a temporary then returns it by value (r-value reference in C++11, by value in C++03).

Obviously with r-value references you have more examples like in your case you can return meow().append( "purr")