Avoid temporary objects

Avoid temporary objects

In the same way that temporary integer variables are used to compute an integer expression, so too are temporary objects used in non-trivial expressions involving class objects. For example, consider this code where the Complex class has defined the + and = operators:

    Complex c1,c2,c3;
    c1 = c2 + c3;

This is likely to create a temporary Complex object as the result of the addition, and this temporary object is then passed as an operand to the = operator. In other words, the expression is actually evaluated as:

    operator=( c1, operator+(c2, c3) );

A temporary object must be created to store the “+” sub-expression computed for the second argument, and then passed to the “=” operator. Whether the operands to operator= are passed by reference or by value has no effect on whether a temporary is created in this situation (it will only affect the creation of new objects inside the operator= function).

One (rather inelegant) method of avoiding this creation of temporaries is to create a specialized function to handle it:

    void AssignThree(Complex &c1, Complex &c2, Complex & c3);
    ...
    AssignThree(c1,c2,c3); // c1 = c2 + c3;

The function should probably be a friend function to allow efficient access to the data members of the three Complex objects.

The problems with this solution are its very poor style (because the neatness of the use of overloaded operators is lost), and also its non-general character. More complicated expressions will still generate temporaries, unless more special functions are added as friend functions, leading to even worse style. This “cure” is perhaps worse than the disease.

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