Re: [javacpp-project] Returning Strings by value, and passing by value or by const-ref

[Samuel Audet]

Hi,

There is no easy way to differentiate between types declared with const, *, and & at the moment, or their parameter names, but this is something I'm thinking about:

https://github.com/bytedeco/javacpp-presets/issues/608

For now the way to work around this is by using Info.javaText() where required:

https://github.com/bytedeco/javacpp/wiki/Mapping-Recipes#mapping-a-declaration-to-custom-code

In this case, we could use something like this:

infoMap.put(new Info("ClassA::getIt").javaText("@Virtual(true) public native @Const({false, false, true}) @Cast({\"const char*\", \"std::string\"}) String getIt();"));
infoMap.put(new Info("ClassA::getWhich").javaText("@Virtual(true) public native @Const({false, false, true}) @Cast({\"const char*\", \"std::string\"}) String getWhich(@Cast({\"const char*\", \"const std::string&\"}) @StdString arg);"));

We can also replace @Cast({"const char*", "std::string"}) @StdString with your @StdStringVal, for example. Sounds good?

Samuel

On 2/3/19 12:23 AM, Ioannis Moschakis wrote:

Hi all,

This may sound like a dumb question but I've spent so much time and I'm still at a loss of how to do this,

I've got a class like so:

using String = std::string; // just stating that i'm using type aliases in cases that creates a problem

using StringVectorT = std::vector<String>; 

class ClassA

{

public:

   virtual String getIt() const = 0;

   virtual String getWhich(const String& arg) const = 0;

   virtual StringVectorT getAll() const = 0;

}

I'm trying to expose said class using the following config:

public class VacuumConfig implements InfoMapper {

@Properties(..)

public class Config implements InfoMapper {

public void map(InfoMap infoMap) {

      infoMap.put(new Info("std::string", "String").annotations("@StdString").pointerTypes("BytePointer"));

                        infoMap.put(new Info("std::vector<std::string>", "StringVectorT").pointerTypes("StringVector").define());

                         

      infoMap.put(new Info("ClassA").pointerTypes("ClassA").purify(false).virtualize());

     }

}

Since @StdString maps to std::basic_string<char>& the overriding function (created due to virtualize) has an conflicting return type (std::basic_string<char>& instead of String (aka std::string).

I've tried other variations:

• infoMap.put(new Info("std::string", "veltio::vacuum::String").valueTypes("String").pointerTypes("@StdString BytePointer")); No effect, return const char* instead of basic_string
  
• infoMap.put(new Info("std::string", "veltio::vacuum::String").valueTypes("@Cast(\"std::string\") String").pointerTypes("@StdString BytePointer")); This solves the problem for the method getIt but creates a whole lot other problems for StringVector (and any other type of structure e.g. maps) since const char* cannot be cast to std::string plus manual modification of the produced java file is required b/c things like this appear in the declaration of StringVector:
  
•  @Cast("std::string") String[] array = new @Cast("std::string") String[size() < Integer.MAX_VALUE ? (int)size() : Integer.MAX_VALUE];
  
• I've even tried defining this:

   @Documented @Retention(RetentionPolicy.RUNTIME)

   @Target({ElementType.METHOD, ElementType.PARAMETER})

   @Cast({"std::basic_string"}) @Adapter("StringAdapter")

   public @interface StdStringVal {

       String value() default "char";

   }

• This creates a problem for the method accepting const String& as an argument (i.e. getWhich).

I've tried searching in javacpp-presets but could pin down a solution to this..

What I need is a mapping that allows methods that accept strings by-value or by const-ref and return string by-value to be able to work okay, is there a way currently to do this?

[Ioannis Moschakis]

Thanks for the prompt answer Samuel! Yeap
at least I didn't miss anything blatant so all good will try this approach thanks for all the work you've put in!

[Ioannis Moschakis]

Hi Samuel,

Unfortunately, this doesn't work either, getIt gets declared like so:

virtual const char* getIt() const;

The dummy implementation get declared like so:

 const char* super_getLabel() { throw JavaCPP_exception("Cannot call pure virtual function ::getIt()."); }

If I change it to @Cast({\"std::string\", \"std::string\"}) then the declaration is okay but then during the definition of the:

JNIEXPORT jstring JNICALL ..._getIt(JNIEnv* env..)

{

...

rptr = (std::string)(const char*)(dynamic_cast<...>(ptr) != NULL ? ((...)ptr)->super_getName() : ptr->getName())

}

This fails b/c you cannot cast std::string to const char* and even if the casts are removed manually it still needs a .c_str() at the end since rptr is const char*.

I can always opt to create a intermediate method that returns const char* and override that in Java, but if there are any other ideas please let me know.

Thanks again!

[Ioannis Moschakis]

Opps too fast made a few mistakes, corrected below:

On Sunday, February 3, 2019 at 8:52:43 PM UTC+2, Ioannis Moschakis wrote:

Hi Samuel,

Unfortunately, this doesn't work either, getIt gets declared like so:

virtual const char* getIt() const;

The dummy implementation get declared like so:

 const char* super_getIt() { throw JavaCPP_exception("Cannot call pure virtual function ::getIt()."); }

If I change it to @Cast({\"std::string\", \"std::string\"}) then the declaration is okay but then during the definition of the:

JNIEXPORT jstring JNICALL ..._getIt(JNIEnv* env..)

{

...

rptr = (std::string)(const char*)(dynamic_cast<...>(ptr) != NULL ? ((...)ptr)->super_getIt() : ptr->getIt())

[Samuel Audet]

I'm unable to reproduce this issue with either JavaCPP 1.4.4 or 1.4.5-SNAPSHOT.

Could you post a small but complete code snippet that fails?

[Ioannis Moschakis]

Hi Samuel,

Sorry it took me this long to reply, I've attached a zip with the with an example of what is not working as described your previous instructions lead to generator generating code that doesn't compile.

Please let me know if I can do anything more to help, in the mean time I've fixed my problem by using proxy methods that return const char*.

Thanks,

Ioannis

[Samuel Audet]

Hi,

After fixing the Infos to the following, it compiles just fine:

infoMap.put(new Info("TestClass::getIt").javaText("@Virtual(true) public native @Const({false, false, true}) @Cast({\"const char*\", \"std::string\"}) @StdString String getIt();"))
infoMap.put(new Info("TestClass::getWhich").javaText("@Virtual(true) public native @Const({false, false, true}) @Cast({\"const char*\", \"std::string\"}) @StdString String getWhich(@Cast({\"const char*\", \"const std::string&\"}) @StdString String arg);"));

Can you confirm?

Samuel

[Ioannis Moschakis]

Hi Samuel,

Confirmed it works fine now, thanks a lot for all the help!

Cheers,

Ioannis