Compiler Warning (level 1) C4312

The latest version of this topic can be found at Compiler Warning (level 1) C4312.

operation' : conversion from 'type1' to 'type2' of greater size

This warning detects an attempt to assign a 32-bit value to a 64-bit pointer type, for example, casting a 32-bit int or long to a 64-bit pointer.

This can be an unsafe conversion even for pointer values that fit in 32 bits when sign extension occurs. If a negative 32-bit integer is assigned to a 64-bit pointer type, sign extension causes the pointer value to reference a memory address different from the value of the integer.

This warning is only issued for 64-bit compilation targets. For more information, see Rules for Using Pointers.

The following code example generates C4312 when it is compiled for 64-bit targets:

// C4312.cpp  
// compile by using: cl /W1 /LD C4312.cpp  
void* f(int i) {  
   return (void*)i;   // C4312 for 64-bit targets  
}  
  
void* f2(__int64 i) {  
   return (void*)i;   // OK  
}