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Reputation: 1457

Non-const lvalue reference to type - Error in Objective-C++ wrapper when using parameter of type Class

I have two wrapper classes written in Objective-C++ for their equivalent C++ classes. Let's call them OABCClass and OXYZCallbackInterface. Now I have a method in C++ (ABCClass) in which an argument is of a interface- XYZCallbackInterface.

For example :

std::string methodWithArguments(std::string req, CommonNamespace::XYZCallbackInterface &callback);

In my Objective-C++ wrapper class for i.e. OABCClass my method implementation for above mentioned C++ method looks like following:

-(NSString *)methodWithArguments(NSString*)req  callback(OXYZCallbackInterface*)callback {
    std::string res = cppClassVariable->methodWithArguments(req.UTF8String, callback);
}

Here I get error :

Non-const lvalue reference to type 'Common::XYZCallbackInterface' cannot bind to a temporary of type 'Common::XYZCallbackInterface *'

What is the correct way to use it? Any help is appreciated.

  //Objective-C++ side
  #ifndef OXYZCallbackInterface_h
  #define OXYZCallbackInterface_h
  #import <Foundation/Foundation.h>

  //.h
  @interface OXYZCallbackInterface : NSObject

  -(bool)onResponseAvailable:(NSString* )response;

  @end
  #endif /* OXYZCallbackInterface_h */

  //.mm
  #import "OXYZCallbackInterface.h"
  #include "Common/Common.Shared/OXYZCallbackInterface.h"

  using namespace CommonNamespace;

  @implementation OXYZCallbackInterface

  - (instancetype)init
  {
   self = [super init];
     return self;
  }

  -(bool)onResponseAvailable:(NSString* )response{
     bool isResp = _objIPICCallback->onResponseAvailable(response.UTF8String);
    return isResp;
 }

  @end

  //C++ side
  #pragma once

  #include "DataTypes.h"

 namespace CommonNamespace
 {
 class XYZCallbackInterface
 {
 public:
    virtual ~ XYZCallbackInterface() {}
    virtual bool onResponseAvailable(std::string response) = 0;
 };
}

And OABCClass.mm implementation looks like:

-(NSString*) methodWithArguments(NSString*)req  callback(OXYZCallbackInterface*)callback{
   NSString* result = @"";

   _pOABC -> methodWithArguments("", (__bridge XYZCallbackInterface*)callback);

   //  Error : Non-const lvalue reference to type CommonNamespace::XYZCallbackInterface cannot bind to a temporary of type CommonNamespace::XYZCallbackInterface *

   return result;
}

Upvotes: 4

Views: 220

Answers (1)

Ken Thomases
Ken Thomases

Reputation: 90531

You don't show your OXYZCallbackInterface Objective-C++ class, so it's hard to be sure how exactly it "wraps" the XYZCallbackInterface C++ interface. However, it's a sure thing that OXYZCallbackInterface is not a XYZCallbackInterface (in the object-oriented "is-a" sense). An instance of OXYZCallbackInterface is not an instance of XYZCallbackInterface (or any subclass). Therefore, you can't pass a pointer to an instance of OXYZCallbackInterface to a function expecting a reference to an instance of XYZCallbackInterface.

Your wrapper will have to provide a means to unwrap itself and return a pointer or reference to the original XYZCallbackInterface object that it wraps. Let's say it has this signature:

- (XYZCallbackInterface*) original;

Then, you could call the C++ method like so:

 std::string res = cppClassVariable->methodWithArguments(req.UTF8String, *[callback original]);

Upvotes: 1

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