使用SWIG将C ++ char *转换为Java中的char []而不是String [英] Using SWIG to convert C++ char* as char[] in Java instead of String

问题描述

我试图用Java包装以下C ++函数：

I am attempting to wrap the following C++ function with Java:

char* MyClass::to_cstring();

此函数的输出作为Java String对象返回。我想它返回一个 char [] Java数组。我目前使用typemaps.i和std_string.i。有没有办法重写的行为，使std :: string仍然返回作为一个Java String ，但 char * 作为Java char数组返回？

This output of this function is returned as a Java String object. I would like it to return as a char[] Java array. I am currently using "typemaps.i" and "std_string.i". Is there a way to override the behavior such that std::string is still returned as a Java String, but char* is returned as a Java char array?

如何使用Java byte [] 而不是 char [] 因此不必担心在8位C ++字符和Java的16位Unicode之间进行转换？

How about using Java byte[] instead of char[] so there is not need to worry about translating between 8-bit C++ chars and Java's 16-bit Unicode?

推荐答案

为此，您需要将默认的SWIG提供的类型标签替换为您自己的。最简单的方法是只需要写一些Javaglue：

To do this you'll need to replace the default SWIG supplied typemaps with one of your own. The simplest way to do this requires just writing some Java "glue":

%module test

%typemap(jstype) char *to_cstring() "byte[]";
%typemap(javaout) char *to_cstring() {
  return $jnicall.getBytes();
}

%inline %{
char *to_cstring() {
  static char ret[] = "hello world";
  return ret;
}
%}

$ c> getBytes（）后面的默认返回的 String 。

Does exactly what you want by calling getBytes() behind the scenes on the default returned String.

你也可以用你自己的一些JNI来做到这一点，以一个字节数组的形式从你的本地代码中返回：

You can also do this with some JNI of your own to return it as a byte array all the way through from your native code:

%module test

%typemap(jstype) char *to_cstring() "byte[]";
%typemap(jtype) char *to_cstring() "byte[]";
%typemap(javaout) char *to_cstring() {
  return $jnicall;
}
%typemap(jni) char *to_cstring() "jbyteArray";
%typemap(out) char *to_cstring() {
  const size_t len = strlen($1);
  $result = JCALL1(NewByteArray, jenv, len);
  // TODO: check that this succeeded
  JCALL4(SetByteArrayRegion, jenv, $result, 0, len, (const jbyte*)$1);
}

%inline %{
char *to_cstring() {
  static char ret[] = "hello world";
  return ret;
}
%}

这里的区别在于映射到 byte [] 发生在生成的JNI中，而不是在Java glue中。

The difference here being that the mapping to byte[] happens in the generated JNI, not in the Java glue. The glue now just proxies directly to the JNI unchanged.

我可以使用下面的Java来测试和验证这些类型：

I was able to test and verify these typemaps with the following Java:

public class run {
  public static void main(String[] argv) {
    System.loadLibrary("test");
    byte[] ret = test.to_cstring();
    System.out.println(ret.length);
    System.out.println((char)ret[0]);
  }
}

在这两个例子中，类型映射类型（ char * ）和函数 to_cstring（）。您可以调整此类型地图匹配的选择性或多或少。它目前不会更改大多数正常使用，您也可以使用％apply 将typemap复制到不完全匹配的其他情况。

In both these examples the typemaps match on both the return type (char *) and the function to_cstring(). You can adjust this typemap matching to be more or less selective. It currently doesn't change most normal usage, you can also use %apply to copy the typemap to other cases which don't exactly match.

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