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.NET Forum / Languages / Managed C++ / October 2004managed string -> unmanaged string
I use the following code to convert a managed string to an unmanaged one: _TCHAR error[255]; const char* umstring = (const char*)Marshal::StringToHGlobalAnsi (merror).ToPointer(); _tcscpy(error, umstring); Marshal::FreeHGlobal(IntPtr((void*)umstring)); I can't use StringToHGlobalAuto because it only ever returns the first character. I asked why a bit ago, and somebody said you can't use StringToHGlobalAuto because of something I can't remember, but in any case it doesn't work so don't say that. But they also said I need to put my own unicode checking around that, but I can't see what the point would be - as aren't all managed strings unicode anyway? This leads me to believe it must be doing whatever conversions for me I require anyway. Is it? If not, what is the required unicode checking? =?Utf-8?B?Qm9uag==?= wrote: > I use the following code to convert a managed string to an unmanaged > one: In general: For string conversion see: HOW TO: Convert from System::String* to Char* in Visual C++ .NET http://support.microsoft.com/kb/311259/ > _TCHAR error[255]; > const char* umstring = (const char*)Marshal::StringToHGlobalAnsi > (merror).ToPointer(); > _tcscpy(error, umstring); > Marshal::FreeHGlobal(IntPtr((void*)umstring)); This works *only* if you have an ANSI/MBCS project! If your projects is compiled with _UNICODE/UNICODE then it will not work! To support TCHAR you have to do: <code> //#include <vcclr.h> System::String * merror = S"Hello world"; _TCHAR error[255]; #ifdef _UNICODE const __wchar_t __pin * umstring = PtrToStringChars(merror); #else const char* umstring = (const char*)Marshal::StringToHGlobalAnsi (error).ToPointer(); #endif _tcscpy(error, umstring); #ifndef _UNICODE Marshal::FreeHGlobal(IntPtr((void*)umstring)); #endif </code> You should also check the string length!!!!  SignatureGreetings Jochen My blog about Win32 and .NET http://blog.kalmbachnet.de/ Thanks very much for the example, it looks rockin'. Just one question. What does "__pin" do? > =?Utf-8?B?Qm9uag==?= wrote: > [quoted text clipped - 34 lines] > > You should also check the string length!!!! =?Utf-8?B?Qm9uag==?= wrote: > Thanks very much for the example, it looks rockin'. > Just one question. What does "__pin" do? It prevents the pointer from GC. SignatureGreetings Jochen My blog about Win32 and .NET http://blog.kalmbachnet.de/ Jochen, > > Thanks very much for the example, it looks rockin'. > > Just one question. What does "__pin" do? > > It prevents the pointer from GC. Actually, it prevents the GC from _moving_ the object pointed to by the __pin pointer, which is slighly different. SignatureTomas Restrepo tomasr@mvps.org but it won't delete it using one of its own threads though either, no? > Jochen, > [quoted text clipped - 5 lines] > Actually, it prevents the GC from _moving_ the object pointed to by the > __pin pointer, which is slighly different. Bonj, > but it won't delete it using one of its own threads though either, no? That's a different thing altogether... it's not the presence of the pinning pointer that prevents garbage collection, per se, but the fact that the object is reachable through some reference to it. In this case, the pointer you created the pinning pointer from should suffice. In other words, we're talking two different things here: you don't usually pin a pointer to prevent it from getting GC'd, you pin it to prevent it from being *moved*. Quite different, really. SignatureTomas Restrepo tomasr@mvps.org Right, so it doesn't matter what happens to the pointer, as long as the data it points to is still there, but if the data gets moved, doom... And that's what the garbage collector does isn't it. But when the GC 'moves' a managed object legitimately, does it update the pointer to point to the new location? Presumably this is transparent normally in .NET but it can't happen with unmanaged pointers? Cheers > Bonj, > [quoted text clipped - 13 lines] > > Quite different, really. Bonj, > Right, so it doesn't matter what happens to the pointer, as long as the data > it points to is still there, but if the data gets moved, doom... > And that's what the garbage collector does isn't it. It does both... move them, and remove them > But when the GC 'moves' a managed object legitimately, does it update the > pointer to point to the new location? Only managed handles in places that are under the GC control (i.e the managed heap, or the managed stack). It won't update pointers in places it it can't control (like in the CRT heap, etc.) That's the reason to have pinning pointers: ensuring the gc doesn't move the object referenced in memory, so that the pointers you pass to it to unmanaged code remain valid... SignatureTomas Restrepo tomasr@mvps.org > Right, so it doesn't matter what happens to the pointer, as long as the data > it points to is still there, but if the data gets moved, doom... > And that's what the garbage collector does isn't it. Yes, after having deleted unreferneced objects, the GC compacts the heap by moving all the remaining objects in contiguous space. > But when the GC 'moves' a managed object legitimately, does it update the > pointer to point to the new location? It doesn't update pointers, it updates managed references (or handles). In managed C++, those handles are represented by "managed pointers". > Presumably this is transparent > normally in .NET but it can't happen with unmanaged pointers? Yes, that's the whole point of pinning pointers : temporaly prevent the GC to move an object, so that unmanaged code can use the memory without risk of access violation. Arnaud MVP - VC > You should also check the string length!!!! See: Convert from System::String* to TCHAR http://blog.kalmbachnet.de/?postid=18 SignatureGreetings Jochen My blog about Win32 and .NET http://blog.kalmbachnet.de/
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