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/Zc:rvalueCast (Enforce type conversion rules)

  • Article

When the /Zc:rvalueCast option is specified, the compiler correctly identifies an rvalue reference type as the result of a cast operation. Its behavior conforms to the C++11 standard. When the option is unspecified, the compiler behavior is the same as in Visual Studio 2012.

Syntax

/Zc:rvalueCast
/Zc:rvalueCast-

Remarks

If /Zc:rvalueCast is specified, the compiler follows section 5.4 of the C++11 standard and treats only cast expressions that result in non-reference types and cast expressions that result in rvalue references to non-function types as rvalue types. By default, or if /Zc:rvalueCast- is specified, the compiler is non-conforming, and treats all cast expressions that result in rvalue references as rvalues. For conformance, and to eliminate errors in the use of casts, we recommend that you use /Zc:rvalueCast.

By default, /Zc:rvalueCast is off (/Zc:rvalueCast-). The /permissive- compiler option implicitly sets this option, but it can be overridden by using /Zc:rvalueCast-.

Use /Zc:rvalueCast if you pass a cast expression as an argument to a function that takes an rvalue reference type. The default behavior causes compiler error C2664 when the compiler incorrectly determines the type of the cast expression. This example shows a compiler error in correct code when /Zc:rvalueCast isn't specified:

// Test of /Zc:rvalueCast
// compile by using:
// cl /c /Zc:rvalueCast- make_thing.cpp
// cl /c /Zc:rvalueCast make_thing.cpp

#include <utility>

template <typename T>
struct Thing {
   // Construct a Thing by using two rvalue reference parameters
   Thing(T&& t1, T&& t2)
      : thing1(t1), thing2(t2) {}

   T& thing1;
   T& thing2;
};

// Create a Thing, using move semantics if possible
template <typename T>
Thing<T> make_thing(T&& t1, T&& t2)
{
   return (Thing<T>(std::forward<T>(t1), std::forward<T>(t2)));
}

struct Test1 {
   long a;
   long b;

   Thing<long> test() {
      // Use identity casts to create rvalues as arguments
      return make_thing(static_cast<long>(a), static_cast<long>(b));
   }
};

The default compiler behavior may not report error C2102 when appropriate. In this example, the compiler doesn't report an error if the address of an rvalue created by an identity cast is taken when /Zc:rvalueCast is unspecified:

int main() {
   int a = 1;
   int *p = &a;   // Okay, take address of lvalue
                  // Identity cast creates rvalue from lvalue;
   p = &(int)a;   // problem: should cause C2102: '&' requires l-value
}

For more information about conformance issues in Visual C++, see Nonstandard Behavior.

To set this compiler option in the Visual Studio development environment

  1. Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

  2. Select the Configuration Properties > C/C++ > Language property page.

  3. Set the Enforce type conversion rules property to /Zc:rvalueCast or /Zc:rvalueCast-. Choose OK or Apply to save your changes.

See also

/Zc (Conformance)