safe_cast (C++ Component Extensions)

 

For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

The safe_cast operation returns the specified expression as the specified type, if successful; otherwise, throws InvalidCastException.

(There are no remarks for this language feature that apply to all runtimes.)

Syntax

  
[default]:: safe_cast<  
type-id  
>(  
expression  
)  
  

Parameters

Remarks

safe_cast allows you to change the type of a specified expression. In situations where you fully expect a variable or parameter to be convertible to a certain type, you can use safe_cast without a try-catch block to detect programming errors during development. For more information, see Casting (C++/CX).

Syntax

  
[default]:: safe_cast<  
type-id  
>(  
expression  
)  
  

Parameters

type-id
The type to convert expression to. A handle to a reference or value type, a value type, or a tracking reference to a reference or value type.

expression
An expression that evaluates to a handle to a reference or value type, a value type, or a tracking reference to a reference or value type.

Remarks

safe_cast throws InvalidCastException if it cannot convert expression to the type specified by type-id. To catch InvalidCastException, specify the /EH (Exception Handling Model) compiler option, and use a try/catch statement.

Requirements

Compiler option: /ZW

Examples

Example

The following code example demonstrates how to use safe_cast with the Windows Runtime.

// safe_cast_ZW.cpp  
// compile with: /ZW /EHsc  
  
using namespace default;  
using namespace Platform;  
  
interface class I1 {};  
interface class I2 {};  
interface class I3 {};  
  
ref class X : public I1, public I2 {};  
  
int main(Array<String^>^ args) {  
   I1^ i1 = ref new X;  
   I2^ i2 = safe_cast<I2^>(i1);   // OK, I1 and I2 have common type: X  
   // I2^ i3 = static_cast<I2^>(i1);   C2440 use safe_cast instead  
   try {  
      I3^ i4 = safe_cast<I3^>(i1);   // Fails because i1 is not derived from I3.  
   }   
   catch(InvalidCastException^ ic) {  
     wprintf(L"Caught expected exception: %s\n", ic->Message);  
   }  
}  

Output

Caught expected exception: InvalidCastException  

safe_cast allows you to change the type of an expression and generate verifiable MSIL code.

Syntax

  
[cli]:: safe_cast<  
type-id  
>(  
expression  
)  
  

Parameters

type-id
A handle to a reference or value type, a value type, or a tracking reference to a reference or value type.

expression
An expression that evaluates to a handle to a reference or value type, a value type, or a tracking reference to a reference or value type.

Remarks

The expression safe_cast<type-id>(expression) converts the operand expression to an object of type type-id.

The compiler will accept a static_cast in most places that it will accept a safe_cast. However, safe_cast is guaranteed to produce verifiable MSIL, where as a static_cast could produce unverifiable MSIL. See Pure and Verifiable Code (C++/CLI) and Peverify.exe (PEVerify Tool) for more information on verifiable code.

Like static_cast, safe_cast invokes user-defined conversions.

For more information about casts, see Casting Operators.

safe_cast does not apply a const_cast (cast away const).

safe_cast is in the cli namespace. See Platform, default, and cli Namespaces for more information.

For more information on safe_cast, see:

Requirements

Compiler option: /clr

Examples

Example

One example of where the compiler will not accept a static_cast but will accept a safe_cast is for casts between unrelated interface types. With safe_cast, the compiler will not issue a conversion error and will perform a check at runtime to see if the cast is possible

// safe_cast.cpp  
// compile with: /clr  
using namespace System;  
  
interface class I1 {};  
interface class I2 {};  
interface class I3 {};  
  
ref class X : public I1, public I2 {};  
  
int main() {  
   I1^ i1 = gcnew X;  
   I2^ i2 = safe_cast<I2^>(i1);   // OK, I1 and I2 have common type: X  
   // I2^ i3 = static_cast<I2^>(i1);   C2440 use safe_cast instead  
   try {  
      I3^ i4 = safe_cast<I3^>(i1);   // fail at runtime, no common type  
   }   
   catch(InvalidCastException^) {  
      Console::WriteLine("Caught expected exception");  
   }  
}  

Output

Caught expected exception  

Component Extensions for Runtime Platforms

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