Wrapper Objects

David M. Beazley
January 15, 2007


This document describes the functions related to management of wrapper objects. A wrapper object is a low-level data structure used to contain the C/C++ code that is emitted during the wrapping process. It contains not only the emitted code, but information about local variables. These objects are a critical component of almost all SWIG target language modules.

The functions described here are declared in Source/Swig/swigwrap.h. This API is considered to be stable.

Creating and Destroying Wrappers

The following functions create and destroy wrapper objects.

Wrapper *NewWrapper()

Creates a new wrapper object.

void DelWrapper(Wrapper *w)

Destroys a wrapper object.

Wrapper Objects

The Wrapper object returned by NewWrapper() has three public attributes.
typedef struct Wrapper {
    String *def;
    String *locals;
    String *code;
} Wrapper;
The def attribute is a string that holds the function definition line. This line declares the function name, return type, and parameters. Language modules create this declaration by simply printing the appropriate text into this attribute.

The locals attribute is a string that holds the code related to any local variables declaration. Normally, language modules do not emit code to this string directly. They use Wrapper_add_local() or Wrapper_new_local() to do this.

The code attribute is a string that holds code related to the body of the function. Almost all code emitted by SWIG language modules is printed into this attribute.

Creating Local Variables

Perhaps the most useful aspect of Wrapper objects is the management of local variables. When creating a wrapper, it is often necessary to emit local variables related to the API of the target language. In addition to this, typemaps and other aspects of SWIG rely upon their own local variables. The following functions are used to create local variables, but also provide support for renaming variables in order to avoid name clashes.

int Wrapper_add_local(Wrapper *w, const String_or_char *name, const String_or_char *decl)

Adds a new local variable to the wrapper object. name is the name of the local variable. decl is a string containing the actual variable declaration code. For example, if you wanted to declare a variable "int x = 42;", you would set name to "x" and decl to "int x = 42;". On success, the text in decl is added to the locals attribute of w and 0 is returned. -1 is returned if a variable with the given name has already been declared.

int Wrapper_add_localv(Wrapper *w, const String_or_char *name, ...)

The same as Wrapper_add_local() except that instead of passing a single string for the declaration, a NULL-terminated list of strings can be passed. These strings are joined together when producing the output. This convention turns out to be fairly useful since language modules often create their output into pieces.

char * Wrapper_new_local(Wrapper *w, const String_or_char *name, const String_or_char *decl)

The same as Wrapper_add_local() except that if a local variable with the given name already exists, this function picks a new name and adds the declaration using the new name. The actual name used for the variable is returned. This function is used when generating code originating from typemaps. For instance, if a typemap declares a local variable, that variable might have to be renamed if the same typemap is used more than once in the same function.

char * Wrapper_new_localv(Wrapper *w, const String_or_char *name,...)

The same as Wrapper_new_localv(), but accepts a NULL-terminated list of strings as code output.

int Wrapper_check_local(Wrapper *w, const String_or_char *name)

Checks to see if a local variable with name name has been declared. Returns 1 if the local is defined, 0 otherwise.


void Wrapper_print(Wrapper *w, File *f)

This function is used to format a wrapper function for output. The formatted wrapper function is emitted to f which may be any file-like object including a FILE * object or a String * object. When emitting the wrapper, the code printed to the wrapper object is automatically formatted. By default, the formatting is done according to a "pretty printing" style in which lines are split onto multiple lines and indented according to reasonable C formatting rules. This produces code that is moderately readable should you want to look at the wrapper code output. An alternative output mode is "compact printing" in which lines are collected and compacted. This may result in multiple C statements appearing on the same line. This mode is sometimes used when the size of a wrapper file is too large for certain compilers. For example, some compilers might impose a limit of 65536 lines per source file.

void Wrapper_compact_print_mode_set(int flag)

Sets the output mode of the Wrapper_print() function. If flag is set to 1, then wrapper code is formatted to be compact.

void Wrapper_pretty_print(String *str, File *f)

Utility function that reformats a string containing C/C++ code and outputs it to the file-like object f. The formatting process indents the code and structures it according to reasonable C formatting rules.

void Wrapper_compact_print(String *str, File *f)

Utility function that reformats a string containing C/C++ code and outputs it to the file-like object f. The formatting process tries to make the code as compact as possible, without going completely overboard. For example, multiple C statements may be combined onto a single line and braces may be aligned to not use up extra lines.

An Example

Here is a simple example of how these functions are used. Suppose you wanted to emit the following C function:
void foo(int n) {
   int i;
   for (i = 0; i < n; i++) {
       printf("%d\n", i);
Here is code that generates the above function:
Wrapper *w = NewWrapper();
Printf(w->def,"void foo(int n) {");
Wrapper_add_local(w,"n","");         /* parameter n */
Wrapper_add_local(w,"i", "int i;");  /* local i */
Printv(w->code,"for (i = 0; i < n; i++) {",
               "}\n", NIL);

/* Emit wrapper code */
Within different language modules, this process is obviously much more involved. However, this example shows the basic idea of how C/C++ code is prepared for output.