德尔福柏林10.1分零缺失异常

Question

I am surprised not the get division by zero exception. How do I get it back?

Berlin 10.1 very recent install, new project,

procedure TForm1.Button1Click(Sender: TObject);
var
  a: Double;
begin
  a := 5/0;                     // No exception
  ShowMessage(a.ToString);      // -> 'INF'
end;

Answer 1

a := 5/0;

The expression 5/0 is, in technical language terms, a constant expression .

A constant expression is an expression that the compiler can evaluate without executing the program in which it occurs. Constant expressions include numerals; character strings; true constants; values of enumerated types; the special constants True, False, and nil; and expressions built exclusively from these elements with operators, typecasts, and set constructors.

As such this expression is evaluated by the compiler, and not at runtime. So its evaluation is determined by compile time rules and cannot be influenced by runtime floating point unit exception masks.

And those rules state that positive value divided by zero is equal to +INF , a special IEEE754 value. If you change the expression to have at least one argument that is not a constant expression then it will be evaluated at runtime, and a divide by zero exception will be raised.

Answer 2

You can control which floating point exceptions are triggered using the SetExceptionMask function in Math unit: http://docwiki.embarcadero.com/Libraries/Tokyo/en/System.Math.SetExceptionMask

To disable all floating point exceptions use:

SetExceptionMask(exAllArithmeticExceptions);

To enable all floating point exception use:

SetExceptionMask([]);

Also note that in your example code the compiler will determine the value at compile time (since the expression is constant) so it will not trigger an exception regardless of the value passed to SetExceptionMask. To trigger exception you need a slightly more complex example, something like this:

program test;
uses Math;
var
  xx,yy: double;
begin
SetExceptionMask([]);
xx := 1;
yy := 0;
halt(round(xx/yy));
end.