SWIG 3 中文手冊——10. 參數處理

目錄

• 10 參數處理
  • 10.1

    typemaps.i

    庫
    • 10.1.1 引言
    • 10.1.2 輸入參數
    • 10.1.3 輸出參數
    • 10.1.4 輸入 / 輸出參數
    • 10.1.5 使用不同的名稱
  • 10.2 對輸入值施加限制
    • 10.2.1 簡單限制的例子
    • 10.2.2 限制方法
    • 10.2.3 對新的資料類型應用限制

In Chapter 3, SWIG's treatment of basic datatypes and pointers was described. In particular, primitive types such as

int

and

double

are mapped to corresponding types in the target language. For everything else, pointers are used to refer to structures, classes, arrays, and other user-defined datatypes. However, in certain applications it is desirable to change SWIG's handling of a specific datatype. For example, you might want to return multiple values through the arguments of a function. This chapter describes some of the techniques for doing this.

第 3 章介紹了 SWIG 對基本資料類型和指針的處理。特别地，原始類型（諸如

int

和

double

）被映射到目智語言中的相應類型。對于其他所有類型，都使用指針來引用結構體、類、數組和其他使用者定義的資料類型。但是，在某些應用程式中，希望更改 SWIG 對特定資料類型的處理。例如，你可能想通過函數的參數傳回多個值。本章介紹了一些執行此類操作的技術。

typemaps.i

This section describes the

typemaps.i

library file--commonly used to change certain properties of argument conversion.

本節描述

typemaps.i

庫檔案——通常用于更改參數轉換的某些屬性。

Suppose you had a C function like this:

假設你有如下 C 函數：

void add(double a, double b, double *result) {
  *result = a + b;
}
           

From reading the source code, it is clear that the function is storing a value in the

double *result

parameter. However, since SWIG does not examine function bodies, it has no way to know that this is the underlying behavior.

One way to deal with this is to use the

typemaps.i

library file and write interface code like this:

通過閱讀源代碼，很明顯該函數将值存儲在

double *result

參數中。但是，由于 SWIG 不檢查函數主體，是以沒有辦法知道函數的底層行為。

一種解決方法是使用

typemaps.i

庫檔案，并編寫如下接口代碼：

// Simple example using typemaps
%module example
%include "typemaps.i"

%apply double *OUTPUT { double *result };
%inline %{
extern void add(double a, double b, double *result);
%}
           

The

%apply

directive tells SWIG that you are going to apply a special type handling rule to a type. The

double *OUTPUT

specification is the name of a rule that defines how to return an output value from an argument of type

double *

. This rule gets applied to all of the datatypes listed in curly braces--in this case

double *result

.

When the resulting module is created, you can now use the function like this (shown for Python):

%apply

指令告訴 SWIG 你将要對某類型應用特殊的類型處理規則。

double *OUTPUT

規範是一個規則的名稱，該規則定義了如何從類型為

double *

的參數傳回輸出值。該規則将應用于大括号中列出的所有資料類型，在這個例子中為

double *result

。

結果子產品建立後，你現在可以這樣使用以下函數（針對 Python）：

>>> a = add(3, 4)
>>> print a
7
>>>
           

In this case, you can see how the output value normally returned in the third argument has magically been transformed into a function return value. Clearly this makes the function much easier to use since it is no longer necessary to manufacture a special

double *

object and pass it to the function somehow.

Once a typemap has been applied to a type, it stays in effect for all future occurrences of the type and name. For example, you could write the following:

在這個例子中，你可以看到通常在第三個參數中傳回的輸出值如何神奇地轉換為函數傳回值。顯然，這使函數更易于使用，因為不再需要制造特殊的

double *

對象，并将其以某種方式傳遞給函數。

一旦将類型映射應用于類型後，它對于以後所有出現的類型和名稱都保持有效。例如，你可以編寫以下内容：

%module example
%include "typemaps.i"

%apply double *OUTPUT { double *result };

%inline %{
extern void add(double a, double b, double *result);
extern void sub(double a, double b, double *result);
extern void mul(double a, double b, double *result);
extern void div(double a, double b, double *result);
%}
...
           

In this case, the

double *OUTPUT

rule is applied to all of the functions that follow.

Typemap transformations can even be extended to multiple return values. For example, consider this code:

在這個例子中，

double *OUTPUT

規則将應用于随後的所有函數。

類型映射轉換甚至可以擴充為多個傳回值。例如，考慮以下代碼：

%include "typemaps.i"
%apply int *OUTPUT { int *width, int *height };

// Returns a pair (width, height)
void getwinsize(int winid, int *width, int *height);
           

In this case, the function returns multiple values, allowing it to be used like this:

在這個例子中，該函數傳回多個值，進而可以像這樣使用它：

>>> w, h = genwinsize(wid)
>>> print w
400
>>> print h
300
>>>
           

It should also be noted that although the

%apply

directive is used to associate typemap rules to datatypes, you can also use the rule names directly in arguments. For example, you could write this:

還應該注意，盡管已經使用

%apply

指令将類型映射規則與資料類型相關聯，但是你也可以直接在參數中使用規則名稱。例如，你可以這樣編寫接口檔案：

// Simple example using typemaps
%module example
%include "typemaps.i"

%{
extern void add(double a, double b, double *OUTPUT);
%}
extern void add(double a, double b, double *OUTPUT);
           

Typemaps stay in effect until they are explicitly deleted or redefined to something else. To clear a typemap, the

%clear

directive should be used. For example:

類型映射将一直有效，直到将其明确删除或重新定義為其他類型為止。要清除類型映射，應使用

%clear

指令。例如：

%clear double *result;      // Remove all typemaps for double *result
           

The following typemaps instruct SWIG that a pointer really only holds a single input value:

以下類型映射告訴 SWIG，指針實際上僅儲存一個輸入值：

int *INPUT
short *INPUT
long *INPUT
unsigned int *INPUT
unsigned short *INPUT
unsigned long *INPUT
double *INPUT
float *INPUT
           

When used, it allows values to be passed instead of pointers. For example, consider this function:

使用時，它允許傳遞值而不是指針。例如，考慮以下函數：

double add(double *a, double *b) {
  return *a + *b;
}
           

Now, consider this SWIG interface:

現在，考慮編寫 SWIG 接口檔案：

%module example
%include "typemaps.i"
...
%{
extern double add(double *, double *);
%}
extern double add(double *INPUT, double *INPUT);
           

When the function is used in the scripting language interpreter, it will work like this:

在腳本語言解釋器中使用該函數時，它将像下面這樣工作：

result = add(3, 4)
           

The following typemap rules tell SWIG that pointer is the output value of a function. When used, you do not need to supply the argument when calling the function. Instead, one or more output values are returned.

以下類型映射規則告訴 SWIG，指針是函數的輸出值。使用時，在調用函數時不需要提供參數。而是傳回一個或多個輸出值。

int *OUTPUT
short *OUTPUT
long *OUTPUT
unsigned int *OUTPUT
unsigned short *OUTPUT
unsigned long *OUTPUT
double *OUTPUT
float *OUTPUT
           

These methods can be used as shown in an earlier example. For example, if you have this C function :

可以如先前示例中所示使用這些方法。例如，如果你具有以下 C 函數：

void add(double a, double b, double *c) {
  *c = a + b;
}
           

A SWIG interface file might look like this :

SWIG 接口檔案可能如下：

%module example
%include "typemaps.i"
...
%inline %{
extern void add(double a, double b, double *OUTPUT);
%}
           

In this case, only a single output value is returned, but this is not a restriction. An arbitrary number of output values can be returned by applying the output rules to more than one argument (as shown previously).

If the function also returns a value, it is returned along with the argument. For example, if you had this:

在這個例子中，僅傳回單個輸出值，但這不是限制。通過将輸出規則應用于多個參數（如上所示），可以傳回任意數量的輸出值。

如果函數還傳回值，則将其與參數一起傳回。例如，如果你有：

extern int foo(double a, double b, double *OUTPUT);
           

The function will return two values like this:

函數将傳回兩個值：

iresult, dresult = foo(3.5, 2)
           

When a pointer serves as both an input and output value you can use the following typemaps :

當指針既用作輸入值又用作輸出值時，可以使用以下類型映射：

int *INOUT
short *INOUT
long *INOUT
unsigned int *INOUT
unsigned short *INOUT
unsigned long *INOUT
double *INOUT
float *INOUT
           

A C function that uses this might be something like this:

有這樣一個 C 函數：

void negate(double *x) {
  *x = -(*x);
}
           

To make

x

function as both and input and output value, declare the function like this in an interface file :

要使

x

既作為函數的輸入值又作為輸出值，請在接口檔案中聲明如下函數：

%module example
%include "typemaps.i"
...
%{
extern void negate(double *);
%}
extern void negate(double *INOUT);
           

Now within a script, you can simply call the function normally :

現在，在腳本中可以輕松地調用函數：

a = negate(3);         # a = -3 after calling this
           

One subtle point of the

INOUT

rule is that many scripting languages enforce mutability constraints on primitive objects (meaning that simple objects like integers and strings aren't supposed to change). Because of this, you can't just modify the object's value in place as the underlying C function does in this example. Therefore, the

INOUT

rule returns the modified value as a new object rather than directly overwriting the value of the original input object.

Compatibility note : The

INOUT

rule used to be known as

BOTH

in earlier versions of SWIG. Backwards compatibility is preserved, but deprecated.

INOUT

規則的一個細微之處是，許多腳本語言對原始對象實施了可變性限制（這意味着簡單的對象，如整數和字元串，不應更改）。是以，你不能像在此示例中基礎 C 函數那樣就地修改對象的值。是以，

INOUT

規則将修改後的值作為新對象傳回，而不是直接覆寫原始輸入對象的值。

注意相容性：在早期版本的 SWIG 中，

INOUT

規則以前被稱為

BOTH

。已保留向後相容性，但已棄用。

As previously shown, the

%apply

directive can be used to apply the

INPUT

,

OUTPUT

, and

INOUT

typemaps to different argument names. For example:

如前所示，

%apply

指令可用于将

INPUT

、

OUTPUT

INOUT

類型映射應用于不同的參數名稱。例如：

// Make double *result an output value
%apply double *OUTPUT { double *result };

// Make Int32 *in an input value
%apply int *INPUT { Int32 *in };

// Make long *x inout
%apply long *INOUT {long *x};
           

To clear a rule, the

%clear

directive is used:

為了清理掉規則，要使用

%clear

指令：

%clear double *result;
%clear Int32 *in, long *x;
           

Typemap declarations are lexically scoped so a typemap takes effect from the point of definition to the end of the file or a matching

%clear

declaration.

類型映射聲明在詞法上是作用域的，是以類型映射從定義點開始到檔案末尾，或比對的

%clear

聲明之前均生效。

In addition to changing the handling of various input values, it is also possible to use typemaps to apply constraints. For example, maybe you want to insure that a value is positive, or that a pointer is non-NULL. This can be accomplished including the

constraints.i

library file.

除了更改對各種輸入值的處理之外，還可以使用類型映射來應用限制。例如，也許你想確定一個值是正數，或者一個指針是非 NULL 的。這可以通過包含

constraints.i

庫檔案來完成。

The constraints library is best illustrated by the following interface file :

以下接口檔案是對

constraints.i

庫最好的說明：

// Interface file with constraints
%module example
%include "constraints.i"

double exp(double x);
double log(double POSITIVE);         // Allow only positive values
double sqrt(double NONNEGATIVE);     // Non-negative values only
double inv(double NONZERO);          // Non-zero values
void   free(void *NONNULL);          // Non-NULL pointers only
           

The behavior of this file is exactly as you would expect. If any of the arguments violate the constraint condition, a scripting language exception will be raised. As a result, it is possible to catch bad values, prevent mysterious program crashes and so on.

該檔案的行為與你期望的完全一樣。如果任何參數違反限制條件，将引發腳本語言異常。最終，可能會捕獲錯誤的值，防止神秘的程式崩潰，等等。

The following constraints are currently available

下列限制是目前可用的：

POSITIVE                     Any number > 0 (not zero)
NEGATIVE                     Any number < 0 (not zero)
NONNEGATIVE                  Any number >= 0
NONPOSITIVE                  Any number <= 0
NONZERO                      Nonzero number
NONNULL                      Non-NULL pointer (pointers only).
           

The constraints library only supports the primitive C datatypes, but it is easy to apply it to new datatypes using

%apply

. For example :

限制庫僅支援原始 C 資料類型，但使用

%apply

可以很容易地将其應用于新的資料類型。例如 ：

// Apply a constraint to a Real variable
%apply Number POSITIVE { Real in };

// Apply a constraint to a pointer type
%apply Pointer NONNULL { Vector * };
           

The special types of "Number" and "Pointer" can be applied to any numeric and pointer variable type respectively. To later remove a constraint, the

%clear

directive can be used :

特殊類型的

Number

Pointer

可以分别應用于任何數字和指針變量類型。為了以後删除限制，可以使用

%clear

%clear Real in;
%clear Vector *;
           

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