C6299

The new home for Visual Studio documentation is Visual Studio 2017 Documentation on docs.microsoft.com.

The latest version of this topic can be found at C6299.

warning C6299: explicitly comparing a bit field to a Boolean type will yield unexpected results

This warning indicates an incorrect assumption that Booleans and bit fields are equivalent. Assigning 1 to bit fields will place 1 in its single bit; however, any comparison of this bit field to 1 includes an implicit cast of the bit field to a signed int. This cast will convert the stored 1 to a -1 and the comparison can yield unexpected results.

The following code generates this warning:

struct myBits  
{  
  short flag : 1;  
  short done : 1;  
  //other members  
} bitType;  
  
void f( )  
{  
  if (bitType.flag == 1)   
  {  
  // code...  
  }  
}  

To correct this warning, use a bit field as shown in the following code:

void f ()  
{  
  if(bitType.flag==bitType.done)  
  {  
    // code...  
  }  
}