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Compiler Warning (level 4) C4254 

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Error Message

'operator' : conversion from 'type1' to 'type2', possible loss of data

A larger bit field was assigned to a smaller bit field. There could be a loss of data.

This warning is off by default. See Compiler Warnings That Are Off by Default for more information.

The following sample generates C4254:

// C4254.cpp
// compile with: /W4
#pragma warning(default: 4254)

struct X {
   int a : 20;
   int b : 12;
};

int main() {
   X *x = new X();
   x->b = 10;
   x->a = 4;
   x->a = x->b;    // OK
   x->b = x->a;    // C4254
};