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Because a pointer can either address an object or address no object at all, a function cannot safely dereference a pointer before it first confirms that the pointer actually addresses an object For example:
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class X; void manip( X *px ) { // make sure it is nonzero before dereference if ( px != 0 ) // dereference the pointer }
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With a reference parameter, on the other hand, the function does not need to guard against its referring to no object A reference must refer to an object, even if we wish otherwise For example:
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class Type { }; void operate( const Type& p1, const Type& p2 ); int main() { Type obj1; // set obj1 to some value // error: argument for reference parameter cannot be 0 Type obj2 = operate( obj1, 0 ); }
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If a parameter may refer to different objects within a function or if the parameter may refer to no object at all, a pointer parameter must be used One important use of reference parameters is to allow us to implement overloaded operators efficiently while keeping their use intuitive The full discussion of overloaded operators is in 15 To get us started, let's examine the following example, which uses a Matrix class type We would like to support both the addition and assignment operations of two Matrix class objects and allow their use to be as "natural" as for the built-in types:
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Matrix a, b, c; c = a + b;
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The addition and assignment operations for the Matrix class type are implemented using overloaded operators An overloaded operator is a function with a funny name In the case of our addition operator, the function name is operator+ Let's provide a definition for this overloaded operator:
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Matrix // addition returns a Matrix object operator+( // name of overloaded operator Matrix m1, // type of the left-hand operand Matrix m2 // type of the right-hand operand ) { Matrix result; // do the computation in result
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return result; }
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This implementation supports the addition of two Matrix objects, such as
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a + b;
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but unfortunately it is unacceptably inefficient Notice that the parameters of our operator+() are not references; this means that the arguments to operator+() are pass-by-value arguments The contents of the two Matrix objects a and b are copied into the parameter area for the operator+() function Because a Matrix class object is quite large, the time and space costs to allocate and copy such an object into the function parameter area are too high to be acceptable Let's pretend that, to improve the efficiency of our operator function, we decide to declare the parameters as pointers Here is what the new implementation of operator+() looks like:
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// new implementation with pointer parameters Matrix operator+( Matrix *m1, Matrix *m2 ) { Matrix result; // do the computation in result return result; }
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However, this implementation has the following problem: although we have gained efficiency, it is at the expense of an intuitive use of the addition operator The pointer parameters now require that we pass the arguments as addresses of the Matrix objects we want to add Our addition operation must now be programmed as follows:
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&a + &b; // not good, but also not impossible
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Although this is ugly and is likely to result in some programmer complaints about not being user-friendly, attempting to add three objects in a compound expression becomes very difficult:
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// oops: this doesn't work // return type of &a + &b is a Matrix object &a + &b + &c;
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To make the addition of three objects well behaved under a pointer solution, the programmer must write the following:
// ok: this works, but &( &a + &b ) + &c;
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