类型检查和范围检查

Question

在我正在阅读的其中一本教科书中说"在编译时检查类型的兼容性,子范围需要运行时范围检查." 如果我说得对,那么当将新值分配给在某个子范围(子类型)中定义的变量时,必须执行运行时检查.类型不一样吗？为何如此区别？这只是编译器的默认值吗？

Answer 1

确实,从类型的子类型P中分配U不需要检查,因为不存在不匹配(即使子类型与类型具有相同的范围; subtype P is U;完全合法且可能有用!)

另一种方式,从类型到子类型,将涉及检查,至少如果子类型是子范围.

从外部来看,具有不同范围的另一种类型也是如此.但是,有一个微妙的区别; 使用类型/子类型,您可以在不进行转换的情况下将一个分配给另一个,如果没有检查,但是在类型/类型中,您必须在分配之前进行类型转换,并且将在转换时检查任何约束违规,而不是分配.

这可能有助于说明这一点:

procedure Drimades is
   type Upper is range 10 .. 20;
   subtype Part is Upper range 10 .. 15;
   type Lower is new Upper range 10 .. 15;

   procedure Assign_U_To_P (U : Upper; To : out Part) is
   begin
      To := U;  -- requires check
   end Assign_U_To_P;

   procedure Assign_P_To_U (P : Part; To : out Upper) is
   begin
      To := P;  -- no check needed
   end Assign_P_To_U;

   procedure Assign_U_To_L (U : Upper; To : out Lower) is
   begin
      To := Lower (U);  -- requires check
   end Assign_U_To_L;

   procedure Assign_L_To_U (L : Lower; To : out Upper) is
   begin
   To := Upper (L);  -- no check required
   end Assign_L_To_U;

   U : Upper;
   P : Part;
   L : Lower;
begin
   Assign_U_To_P (20, P);
   Assign_P_To_U (15, U);
   Assign_U_To_L (20, L);
   Assign_L_To_U (15, U);
end Drimades;

用GNAT编译这个并且开关-gnatG产生这个中间表示,不太难以解释我希望:

procedure drimades is
   type drimades__upper is range 10 .. 20;
   [type drimades__TupperB is new short_short_integer]
   freeze drimades__TupperB []
   subtype drimades__part is drimades__upper range 10 .. 15;
   [type drimades__TlowerB is new drimades__TupperB]
   freeze drimades__TlowerB []
   type drimades__lower is new drimades__upper range 10 .. 15;

   procedure drimades__assign_u_to_p (u : drimades__upper; to : out
     drimades__part) is
   begin
      [constraint_error when
        not (u in 10 .. 15)
        "range check failed"]
      to := u;
      return;
   end drimades__assign_u_to_p;

   procedure drimades__assign_p_to_u (p : drimades__part; to : out
     drimades__upper) is
   begin
      to := p;
      return;
   end drimades__assign_p_to_u;

   procedure drimades__assign_u_to_l (u : drimades__upper; to : out
     drimades__lower) is
   begin
      [constraint_error when
        not (u in 10 .. 15)
        "range check failed"]
      to := drimades__lower(u);
      return;
   end drimades__assign_u_to_l;

   procedure drimades__assign_l_to_u (l : drimades__lower; to : out
     drimades__upper) is
   begin
      to := drimades__upper(l);
      return;
   end drimades__assign_l_to_u;

   u : drimades__upper;
   p : drimades__part;
   l : drimades__lower;
begin
   drimades__assign_u_to_p (20, p);
   drimades__assign_p_to_u (15, u);
   drimades__assign_u_to_l (20, l);
   drimades__assign_l_to_u (15, u);
   return;
end drimades;