C++/CLI : Why can't I pass Strings by reference?

Question

Why doesn't Microsoft's C++/CLI allow me to pass strings by reference? I received the following error:

C3699: '&': cannot use this indirection on type 'System::String'

Solution 1

Sounds like you are using Managed C++, which is a bastardised C++ used with the .NET Framework.

in Managed C++, I believe the syntax you are looking for is System::String^. The reason for this is that since managed types are garbage collected by .NET Framework, you aren't allowed to create 'regular' references since the GC needs to track all the references to a specific variable to know when it is safe to free it.

Solution 2

First of all, there are really two Microsoft-specific C++ dialects for .NET: the older "Managed C++" (Visual Studio 2002 and 2003) and C++/CLI (Visual Studio 2005 and later).

In C++/CLI, System::String^ is a .NET reference to a string; some authors call this a "tracking pointer" to compare and contrast it with a normal C++ pointer. As in C++, you can pass .NET references "by reference", but instead of using &, you use %, as in:

void makeStr(System::String^ %result) {
   result = gcnew System::String("abc");
}

Solution 3

It looks like you are using Managed C++. You should use System::String^ instead.

Comments

[configurator]

I added the C++-cli tag because it looks like that's what you're using here.

[jwadsack]

Take a look at Microsoft's tutorial on Data Marshaling for an in-depth discussion on how memory is managed between native C/C++ and Managed C++. msdn.microsoft.com/en-us/library/ms384317(VS.71).aspx

[user1606191]

Do we explicitly need to delete the variable "result" passed from C# layer or does the GC takes care of deleting it?

[csjoseph]

I think GC will take care of it.