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String (C++ Component Extensions)

The Visual C++ compiler supports strings, which are objects that represent text as a sequence of characters. Visual C++ supports string variables, whose value is implicit, and literals, whose value is an explicit quoted string.

The Windows Runtime and common language runtime represent strings as objects whose allocated memory is managed automatically. That is, you are not required to explicitly discard the memory for a string when the string variable goes out of scope or your application ends. To indicate that the lifetime of a string object is to be managed automatically, declare the string type with the handle-to-object (^) modifier.

The Windows Runtime architecture requires Visual C++ to implement the String data type in the Platform namespace. For your convenience, Visual C++ also provides the string data type, which is a synonym for Platform::String, in the default namespace.

// compile with /ZW
using namespace Platform;
using namespace default;
   Platform::String^ MyString1 = "The quick brown fox";
   String^ MyString2 = "jumped over the lazy dog.";
   String^ MyString3 = "Hello, world!";

For more information and examples about strings, see Platform::String, std::wstring, and Literals (Platform)

Compiler option: /ZW

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 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.

Note Note

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

For more information see C++ String Literals.

Compiler option: /clr

Example

The following code example demonstrates concatenating and comparing strings.

// string_operators.cpp
// compile with: /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 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

Example

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 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

Example

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

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