In C++/CLI, how do I declare and call a function with an 'out' parameter?

Question

I have a function which parses one string into two strings. In C# I would declare it like this:

void ParseQuery(string toParse, out string search, out string sort) {     ... } 

and I'd call it like this:

string searchOutput, sortOutput; ParseQuery(userInput, out searchOutput, out sortOutput); 

The current project has to be done in C++/CLI. I've tried

using System::Runtime::InteropServices;  ...  void ParseQuery(String ^ toParse, [Out] String^ search, [Out] String^ sort) {     ... } 

but if I call it like this:

String ^ searchOutput, ^ sortOutput; ParseQuery(userInput, [Out] searchOutput, [Out] sortOutput); 

I get a compiler error, and if I call it like this:

String ^ searchOutput, ^ sortOutput; ParseQuery(userInput, searchOutput, sortOutput); 

then I get an error at runtime. How should I declare and call my function?

Answer 1

C++/CLI itself doesn't support a real 'out' argument, but you can mark a reference as an out argument to make other languages see it as a real out argument.

You can do this for reference types as:

void ReturnString([Out] String^% value) {    value = "Returned via out parameter"; }  // Called as String^ result; ReturnString(result); 

And for value types as:

void ReturnInt([Out] int% value) {    value = 32; }  // Called as int result; ReturnInt(result); 

The % makes it a 'ref' parameter and the OutAttribute marks that it is only used for output values.