How to convert from System::String* to Char* in Visual C++

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This article was previously published under Q311259

This article refers to the following Microsoft .NET Framework Class Library namespaces:

System::Runtime::InteropServices
Msclr::interop

SUMMARY

This article discusses several ways to convert from System::String* to char* by using the following:

Managed extensions for C++ in Microsoft Visual C++ .NET 2002 and in Microsoft Visual C++ .NET 2003
C++/CLI in Microsoft Visual C++ 2005 and in Microsoft Visual C++ 2008

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Method 1

PtrToStringChars gives you an interior pointer to the actual String object. If you pass this pointer to an unmanaged function call, you must first pin the pointer to ensure that the object does not move during an asynchronous garbage collection process:

//#include <vcclr.h>
System::String * str = S"Hello world\n";
const __wchar_t __pin * str1 = PtrToStringChars(str);
wprintf(str1);

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Method 2

StringToHGlobalAnsi copies the contents of a managed String object into native heap, and then converts it into American National Standards Institute (ANSI) format on the fly. This method allocates the required native heap memory:

//using namespace System::Runtime::InteropServices;
System::String * str = S"Hello world\n";
char* str2 = (char*)(void*)Marshal::StringToHGlobalAnsi(str);
printf(str2);
Marshal::FreeHGlobal(str2);

Note In Visual C++ 2005 and in Visual C++ 2008, you must add the common language runtime support compiler option (/clr:oldSyntax) to successfully compile the previous code sample. To add the common language runtime support compiler option, follow these steps:

Click Project, and then click ProjectName Properties.

Note ProjectName is a placeholder for the name of the project.
Expand Configuration Properties, and then click General.
In the right pane, click to select Common Language Runtime Support, Old Syntax (/clr:oldSyntax) in the Common Language Runtime support project settings.
Click Apply, and then click OK.

For more information about common language runtime support compiler options, visit the following Microsoft Developer Network (MSDN) Web site:

http://msdn2.microsoft.com/en-us/library/k8d11d4s.aspx (http://msdn2.microsoft.com/en-us/library/k8d11d4s.aspx)

These steps apply to the whole article.

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Method 3

The VC7 CString class has a constructor that takes a managed String pointer and loads the CString with its contents:

//#include <atlstr.h>
System::String * str = S"Hello world\n";
CString str3(str); 
printf(str3);

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Method 4

Visual C++ 2008 introduces the marshal_as<T> marshal help class and the marshal_context() marshal helper class.

//#include <msclr/marshal.h>
//using namespace msclr::interop;
marshal_context ^ context = gcnew marshal_context();
const char* str4 = context->marshal_as<const char*>(str);
puts(str4);
delete context;

Note This code does not compile by using managed extensions for C++ in Visual C++ .NET 2002 or in Visual C++ .NET 2003. It uses the new C++/CLI syntax that was introduced in Visual C++ 2005 and the new msclr namespace code that was introduced in Visaul C++ 2008. To successfully compile this code, you must use the /clr C++ compiler switch in Microsoft Visual C++ 2008.

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Managed Extensions for C++ sample code (Visual C++ 2002 or Visual C++ 2003)

//compiler option: cl /clr  
#include <vcclr.h>
#include <atlstr.h>
#include <stdio.h>
#using <mscorlib.dll>
using namespace System;
using namespace System::Runtime::InteropServices;

int _tmain(void)
{
   	System::String * str = S"Hello world\n";

	//method 1
	const __wchar_t __pin * str1 = PtrToStringChars(str);
	wprintf(str1);	

	//method 2
	char* str2 = (char*)(void*)Marshal::StringToHGlobalAnsi(str);
	printf(str2);
	Marshal::FreeHGlobal(str2);

	//method 3
	CString str3(str); 
	wprintf(str3);

    return 0;
}

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C++/CLI sample code (Visual C++ 2005 and Visual C++ 2008)

//compiler option: cl /clr 

#include <atlstr.h>
#include <stdio.h>
#using <mscorlib.dll>

using namespace System;
using namespace System::Runtime::InteropServices;

#if _MSC_VER > 1499 // Visual C++ 2008 only
#include <msclr/marshal.h>
using namespace msclr::interop;
#endif 

int _tmain(void)
{
		     System::String ^ str = "Hello world\n";

	     /method 1
	     pin_ptr<const wchar_t> str1 = PtrToStringChars(str);
     	wprintf(str1);	

	     //method 2
	     char* str2 = (char*)Marshal::StringToHGlobalAnsi(str).ToPointer();
     	printf(str2);
	     Marshal::FreeHGlobal((IntPtr)str2);

	     //method 3
	     CString str3(str); 
	     wprintf(str3);

     	//method 4
#if _MSC_VER > 1499 // Visual C++ 2008 only
	     marshal_context ^ context = gcnew marshal_context();
	     const char* str4 = context->marshal_as<const char*>(str);
     	puts(str4);
	     delete context;
#endif

	return 0;
}

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REFERENCES

For other top-hit Visual C++ .NET Microsoft Knowledge Base articles, visit the following Microsoft Web site:

http://support.microsoft.com/default.aspx?xmlid=fh%3BEN-US%3Bvcnet (http://support.microsoft.com/default.aspx?xmlid=fh%3ben-us%3bvcnet)

For more general information about Visual C++ .NET, visit the following Microsoft Usenet newsgroup:

Microsoft.public.dotnet.languages.vc (http://msdn.microsoft.com/newsgroups/default.aspx?query=Microsoft.public.dotnet.languages.vc+&dg=&cat=en-us-msdn&lang=en&cr=US&pt=&catlist=774F24A2-F71F-425F-AC2B-DC48AB0DA5C9&dglist=&ptlist=&exp=&sloc=en-us)

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