| Problem |
Profiling shows that function calls are taking much
of the real time.
|
| Context |
During design, when establishing interfaces for functions
(member functions and otherwise), or when improving
the performance of an existing program.
|
| Forces |
You want to treat objects like built-in data types, which
means in part that you want to assume that they have efficient
implementations. Yet efficiency in the C tradition comes from
pointers, particularly in parameter lists, to defer the copying of
data until it is actually used. An innocent-looking function call
can trigger one or more constructor calls for each actual parameter.
And those constructors may in turn allocate dynamic storage! Most function parameters are actually const parameters in good object-oriented programs, whether the function prototypes capture that or not. Except under the most arcane conditions, a function can assume that the lifetime of its actual parameters bounds the lifetime of the function's own activation record.
|
| Solution |
Use call-by-reference instead of call-by-value. Make
sure function prototypes capture the semantics of const where
applicable.
|
|
Resulting context |
Constructors won't fire for function parameters, and the real-time
overhead is reduced while maintaining equivalent functionality.
|
| [Source: James Coplien, "After all, we can't ignore efficiency", C++ Report, May 96, p68] |