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Compiler Warning (level 4) C4239


The latest version of this topic can be found at Compiler Warning (level 4) C4239.

nonstandard extension used : 'token' : conversion from 'type' to 'type'

This type conversion is not allowed by the C++ standard, but it is permitted here as an extension. This warning is always followed by at least one line of explanation describing the language rule being violated.

The following sample generates C4239.

// C4239.cpp  
// compile with: /W4 /c  
struct C {  
   C() {}  
void func(void) {  
   C & rC = C();   // C4239  
   const C & rC2 = C();   // OK  

Conversion from integral type to enum type is not strictly allowed.

The following sample generates C4239.

// C4239b.cpp  
// compile with: /W4 /c  
enum E { value };   
struct S {   
   E e : 2;   
} s = { 5 };   // C4239   
// try the following line instead  
// } s = { (E)5 };