Wrapper function

A wrapper function is a subroutine in a software library or a computer program whose main purpose is to call a second subroutine^[1] or a system call with little or no additional computation.

Purpose

Wrapper functions are a means of delegation and can be used for a number of purposes.

Programming convenience

Wrapper functions can be used to make writing computer programs easier. An example of this is the MouseAdapter and similar classes in the Java AWT library.^[2] Wrapper functions are useful in the development of applications that use third-party library functions. A wrapper can be written for each of the third party functions and used in the native application. In case the third party functions change or are updated, only the wrappers in the native application need to be modified as opposed to changing all instances of third party functions in the native application.

Adapting class/object interfaces

Wrapper functions can be used to adapt an existing class or object to have a different interface. This is especially useful when using existing library code.

Code testing

Wrapper functions can be used to write error checking routines for pre-existing system functions without increasing the length of a code by a large amount by repeating the same error check for each call to the function.^[3] All calls to the original function can be replaced with calls to the wrapper, allowing the programmer to forget about error checking once the wrapper is written. A test driver is a kind of wrapper function that exercises a code module, typically calling it repeatedly, with different settings or parameters, in order to rigorously pursue each possible path. It is not deliverable code, but is not throwaway code either, being typically retained for use in regression testing. An interface adaptor is a kind of wrapper function that simplifies, tailors, or amplifies the interface to a code module, with the intent of making it more intelligible or relevant to the user. It may rename parameters, combine parameters, set defaults for parameters, and the like.

Multiple inheritance

In a programming language that does not support multiple inheritance of base classes, wrapper functions can be used to simulate it. Below is an example of part of a Java class that "inherits" from LinkedList and HashSet.

public class StackSet implements Stack, Set {

    private LinkedList stack;
    private HashSet set;

    public boolean push(Object o) {
        if (set.add(o)) return stack.push(o);
        else return false;
    }

    public Object pop() {
        Object o = stack.pop();
        set.remove(o);
        return o;
    }

    public boolean contains(Object o) {
        return set.contains(o);
    }

}

Library functions and system calls

Many library functions, such as those in the C Standard Library, act as interfaces for abstraction of system calls. The fork and execve functions in glibc are examples of this. They call the lower-level fork and execve system calls, respectively.

This may lead to incorrectly using the terms "system call" and "syscall" to refer to higher-level library calls rather than the similarly named system calls, which they wrap.

See also

• Wrapper library
• Adapter pattern
• Decorator pattern
• Delegation (programming)
• Forwarding (object-oriented programming)
• Language binding wrapper to another language
• SWIG automatic wrapper generator
• Nested function
• Partial application

References

1. Reselman, Bob; Peasley, Richard; Pruchniak, Wayne (1998). Using Visual Basic 6. Que. p. 446. ISBN 9780789716330.
2. The Java Tutorials
3. Stevens, Richard; Fenner, Bill; Rudoff; Andrew M. (2003). UNIX Network Programming. Addison-Wesley. pp. 5–6,29. ISBN 9780131411555.