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Some guidelines for using expression-oriented classes

The fact that C++ allows operators to be overloaded for user-defined classes can make programming with library classes like Integer, String, and so on very convenient. However, it is worth becoming familiar with some of the inherent limitations and problems associated with such operators.

Many operators are constructive, i.e., create a new object based on some function of some arguments. Sometimes the creation of such objects is wasteful. Most library classes supporting expressions contain facilities that help you avoid such waste.

For example, for Integer a, b, c; ...; c = a + b + a;, the plus operator is called to sum a and b, creating a new temporary object as its result. This temporary is then added with a, creating another temporary, which is finally copied into c, and the temporaries are then deleted. In other words, this code might have an effect similar to Integer a, b, c; ...; Integer t1(a); t1 += b; Integer t2(t1); t2 += a; c = t2;.

For small objects, simple operators, and/or non-time/space critical programs, creation of temporaries is not a big problem. However, often, when fine-tuning a program, it may be a good idea to rewrite such code in a less pleasant, but more efficient manner.

For builtin types like ints, and floats, C and C++ compilers already know how to optimize such expressions to reduce the need for temporaries. Unfortunately, this is not true for C++ user defined types, for the simple (but very annoying, in this context) reason that nothing at all is guaranteed about the semantics of overloaded operators and their interrelations. For example, if the above expression just involved ints, not Integers, a compiler might internally convert the statement into something like c = a; c += b; c+= a; , or perhaps something even more clever. But since C++ does not know that Integer operator += has any relation to Integer operator +, A C++ compiler cannot do this kind of expression optimization itself.

In many cases, you can avoid construction of temporaries simply by using the assignment versions of operators whenever possible, since these versions create no temporaries. However, for maximum flexibility, most classes provide a set of "embedded assembly code" procedures that you can use to fully control time, space, and evaluation strategies. Most of these procedures are "three-address" procedures that take two const source arguments, and a destination argument. The procedures perform the appropriate actions, placing the results in the destination (which is may involve overwriting old contents). These procedures are designed to be fast and robust. In particular, aliasing is always handled correctly, so that, for example add(x, x, x); is perfectly OK. (The names of these procedures are listed along with the classes.)

For example, suppose you had an Integer expression a = (b - a) * -(d / c);

This would be compiled as if it were Integer t1=b-a; Integer t2=d/c; Integer t3=-t2; Integer t4=t1*t3; a=t4;

But, with some manual cleverness, you might yourself some up with sub(a, b, a); mul(a, d, a); div(a, c, a);

A related phenomenon occurs when creating your own constructive functions returning instances of such types. Suppose you wanted to write function Integer f(const Integer& a) { Integer r = a; r += a; return r; }

This function, when called (as in a = f(a); ) demonstrates a similar kind of wasted copy. The returned value r must be copied out of the function before it can be used by the caller. In GNU C++, there is an alternative via the use of named return values. Named return values allow you to manipulate the returned object directly, rather than requiring you to create a local inside a function and then copy it out as the returned value. In this example, this can be done via Integer f(const Integer& a) return r(a) { r += a; return; }

A final guideline: The overloaded operators are very convenient, and much clearer to use than procedural code. It is almost always a good idea to make it right, then make it fast, by translating expression code into procedural code after it is known to be correct.

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