Function Objects in Java

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Function Objects
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Our min() function is a good example of both the power and the limitation of the template mechanism
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template <typename Type> const Type& min( const Type *p, int size ) { int minIndex = 0; for ( int ix = 1; ix < size; ++ix ) if ( p[ ix ] < p[ minIndex ] ) minIndex = ix; return p[ minIndex ]; }
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The power comes from the ability to define a single instance of min() that can be instantiated for an infinite number of types The limitation is that although min() can be instantiated for an infinite number of types, it is not universally applicable to all types The constraint centers on the use of the less-than operator In one case, the underlying type may not support the less-than operator An Image class, for example, may not provide an implementation of a less-than operator, although we may be unaware of that and wish to discover the minimum frame number of the array of Image objects An attempt to instantiate min() with an array of our Image class, however, results in a compile-time error:
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error: invalid types applied to the < operator: Image < Image
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In a second case, a less-than operator exists, but the provided semantics are inappropriate For example, if we wish to discover a minimum string but wish to consider only letters of the alphabet and not be case-sensitive, then the less-than operator, although it is supported, supports the wrong semantics The traditional solution is to parameterize the comparison operator, in this case declaring a pointer to function taking two arguments and returning a value of type bool:
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template < typename Type, bool (*Comp)(const Type&, const Type&)> const Type& min( const Type *p, int size, Comp comp ) { int minIndex = 0; for ( int ix = 1; ix < size; ++ix ) if ( Comp( p[ ix ], p[ minIndex ] )) minIndex = ix; return p[ minIndex ]; }
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This solution, together with our first implementation using the built-in less-than operator, provides general support for any type, including our Image class should we care to implement its semantics in terms of two Images The primary performance drawback of a pointer to function is that its indirect invocation prevents it from being inlined An alternative parameterization strategy to that of a pointer to function is a function object (we saw a number of examples in the previous section) A function object is a class that overloads the function call operator (operator()) The operator encapsulates what normally would be implemented as a function A function object is typically passed as an argument to a generic algorithm, although we can also define individual function objects For example, if the class AddImages were defined as a function object to take two images, composite them (that is, add the two together) and then return a new image, we might declare an instance of it as follows:
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To have the function object perform its operation, we apply the call operator, providing the necessary Image class operands For example:
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Image im1("foregroundtiff"), im2("backgroundtiff"); // // invokes Image AddImages::operator()(const Image&,const Image&); Image new_image = AI( im1, im2 );
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The benefits of a function object over a pointer to function are twofold First, if the overloaded call operator is an inline function, the compiler is able to perform the inlining, providing a possibly significant efficiency gain Second, the function object can hold an arbitrary amount of additional data, either cached results or data to help in the current operation Here is our revised implementation of min() (note that a pointer to function can also be passed using this declaration, but without any checking of its prototype):
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template < typename typename const Type& min( const Type *p, { int minIndex = for ( int ix =
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Type, Comp > int size, Comp comp ) 0; 1; ix < size; ++ix )
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