System::String Handling in Visual C++ 

This topic discusses how the Visual C++ compiler processes string literals when compiling with /clr.

Remarks

When compiling with /clr, the compiler will convert string literals to strings of type String. To preserve backward compatibility with existing code there are two exceptions to this:

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

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

The compiler also has built-in support for three 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 compiling with /clr:oldSyntax, string literals will not be converted to String.

For more information see:

• C++ String Literals

• /clr (Common Language Runtime Compilation)

Example

// string_operators.cpp
// compile with: /clr
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 returning 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");
}

Output

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

This 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 with: /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);
}

Output

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 with: /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
}

Output

char *
String^ str
System.SByte*
System.String

See Also

Concepts

New C++ Language Features