Declaring an object before initializing it in c++

Question

Is it possible to declare a variable in c++ without instantiating it? I want to do something like this:

Animal a;
if( happyDay() ) 
    a( "puppies" ); //constructor call
else
    a( "toads" );

Basially, I just want to declare a outside of the conditional so it gets the right scope.

Is there any way to do this without using pointers and allocating a on the heap? Maybe something clever with references?

Answer 1

If you want to avoid garbage collection - you could use a smart pointer.

auto_ptr<Animal> p_a;
if ( happyDay() )
    p_a.reset(new Animal( "puppies" ) );
else
    p_a.reset(new Animal( "toads" ) );

// do stuff with p_a-> whatever.  When p_a goes out of scope, it's deleted.

If you still want to use the . syntax instead of ->, you can do this after the code above:

Animal& a = *p_a;

// do stuff with a. whatever

Answer 2

Yes, you can do do the following:

Animal a;
if( happyDay() )
    a = Animal( "puppies" );
else
    a = Animal( "toads" );

That will call the constructors properly.

EDIT: Forgot one thing... When declaring a, you'll have to call a constructor still, whether it be a constructor that does nothing, or still initializes the values to whatever. This method therefore creates two objects, one at initialization and the one inside the if statement.

A better way would be to create an init() function of the class, such as:

Animal a;
if( happyDay() )
    a.init( "puppies" );
else
    a.init( "toads" );

This way would be more efficient.