Was this page helpful?
Your feedback about this content is important. Let us know what you think.
Additional feedback?
1500 characters remaining
Export (0) Print
Expand All
Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

System::String Handling in Visual C++

Updated: October 2009

This topic discusses how the Visual C++ compiler processes string literals when you run it by using the /clr compiler option. To use /clr, you must also use the common language runtime (CLR), C++/CLI syntax and managed objects. For more information about /clr, see /clr (Common Language Runtime Compilation).

When you use /clr, the compiler converts string literals to strings of type String. However, to preserve backward compatibility with existing code, there are these exceptions:

  • Exception handling. When a string literal is thrown, the compiler catches it as a string literal.

  • Template deduction. When a string literal is passed as a template argument, the compiler does not convert it to a String. String literals passed as a generic argument are promoted to String.

The compiler also has built-in support for the following operators, which you can override to customize their behavior:

  • System::String ^ operator +( System::String, System::String);

  • System::String ^ operator +( System::Object, System::String);

  • System::String ^ operator +( System::String, System::Object);

When passed a String, the compiler will box, if necessary, and then concatenate the object (with ToString) with the string.

When you compile by using /clr:oldSyntax, string literals are not converted to String.

NoteNote:

The caret ("^") indicates that the declared variable is a handle to a C++/CLI managed object.

For more information see C++ String Literals.

// string_operators.cpp
// compile by using: /clr
// In the following code, the caret ("^") indicates that the 
// declared variable is a handle to a C++/CLI managed object.
using namespace System;

int main() {
   String ^ a = gcnew String("abc");
   String ^ b = "def";   // same as gcnew form
   Object ^ c = gcnew String("ghi");

   char d[100] = "abc";

   // variables of System::String that return a System::String
   Console::WriteLine(a + b);
   Console::WriteLine(a + c);
   Console::WriteLine(c + a);

   // accessing a character in the string
   Console::WriteLine(a[2]);

   // concatenation of three System::Strings
   Console::WriteLine(a + b + c);

   // concatenation of a System::String and string literal
   Console::WriteLine(a + "zzz");

   // you can append to a System::String ^
   Console::WriteLine(a + 1);
   Console::WriteLine(a + 'a');
   Console::WriteLine(a + 3.1);

   // test System::String ^ for equality
   a += b;
   Console::WriteLine(a);
   a = b;
   if (a == b)
      Console::WriteLine("a and b are equal");

   a = "abc";
   if (a != b)
      Console::WriteLine("a and b are not equal");

   // System:String ^ and tracking reference
   String^% rstr1 = a;
   Console::WriteLine(rstr1);

   // testing an empty System::String ^
   String ^ n;
   if (n == nullptr)
      Console::WriteLine("n is empty");
}
abcdef abcghi ghiabc c abcdefghi abczzz abc1 abc97 abc3.1 abcdef a and b are equal a and b are not equal abc n is empty

The following sample shows that you can overload the compiler-provided operators, and that the compiler will find a function overload based on the String type.

// string_operators_2.cpp
// compile by using: /clr
using namespace System;

// a string^ overload will be favored when calling with a String
void Test_Overload(const char * a) { 
   Console::WriteLine("const char * a"); 
}
void Test_Overload(String ^ a) { 
   Console::WriteLine("String ^ a"); 
}

// overload will be called instead of compiler defined operator
String ^ operator +(String ^ a, String ^ b) {
   return ("overloaded +(String ^ a, String ^ b)");
}

// overload will be called instead of compiler defined operator
String ^ operator +(Object ^ a, String ^ b) {
   return ("overloaded +(Object ^ a, String ^ b)");
}

// overload will be called instead of compiler defined operator
String ^ operator +(String ^ a, Object ^ b) {
   return ("overloaded +(String ^ a, Object ^ b)");
}

int main() {
   String ^ a = gcnew String("abc");
   String ^ b = "def";   // same as gcnew form
   Object ^ c = gcnew String("ghi");

   char d[100] = "abc";

   Console::WriteLine(a + b);
   Console::WriteLine(a + c);
   Console::WriteLine(c + a);

   Test_Overload("hello");
   Test_Overload(d);
}
overloaded +(String ^ a, String ^ b) overloaded +(String ^ a, Object ^ b) overloaded +(Object ^ a, String ^ b) String ^ a const char * a

The following sample shows that the compiler distinguishes between native strings and String strings.

// string_operators_3.cpp
// compile by using: /clr
using namespace System;
int func() {
   throw "simple string";   // const char *
};

int func2() {
   throw "string" + "string";   // returns System::String
};

template<typename T>
void func3(T t) {
   Console::WriteLine(T::typeid);
}

int main() {
   try {
      func();
   }
   catch(char * e) {
      Console::WriteLine("char *");
   }

   try {
      func2();
   }
   catch(String^ str) {
      Console::WriteLine("String^ str");
   }

   func3("string");   // const char *
   func3("string" + "string");   // returns System::String
}
char * String^ str System.SByte* System.String

Compiler option: /clr

Date

History

Reason

October 2009

Added context to associate C++/CLI syntax with the /clr compiler switch.

Customer feedback.

Community Additions

ADD
Show:
© 2015 Microsoft