我正在阅读Ada 中的强类型，重点是单元检查，并决定自己测试这种幼稚的方法：

procedure Example is
  type Meters is new Float;
  type Meters_Squared is new Float; 
  function "*" (Left, Right : Meters) return Meters_Squared is
  begin
    return Meters_Squared(Float(Left)*Float(Right));
  end;
  len_a : Meters := 10.0;
  len_b : Meters := 15.0;
  surface : Meters_Squared;
  len_sum : Meters;
begin
  len_sum := len_a + len_b; -- ok
  surface := len_a * len_b; -- ok
  len_sum := len_a * len_b; -- invalid
end Example;

现在我知道这实际上并不是实用的方法，我只是将其作为一种学习经验来尝试。根据我到目前为止的尝试，我一定遗漏了一些东西，因为当我尝试编译上面列出的示例时，我没有收到任何错误：

$ make example
gcc -c example.adb
gnatmake example.adb
gnatbind -x example.ali
gnatlink example.ali

当我删除重载乘法运算符的函数定义时，它会按预期失败：

$ make example
gcc -c example.adb
example.adb:14:20: expected type "Meters_Squared" defined at line 3
example.adb:14:20: found type "Meters" defined at line 2
make: *** [Makefile:6: example] Error 1

考虑到这一点，我不明白怎么回事，用乘法运算符重载，编译器可确定与surface := len_a * len_b和len_sum := len_a * len_b在同一时间。

回答

function "*" (Left, Right: Meters) return Meters;

function "*" (Left, Right: Meters) return Meters
is abstract;

function "and" (Left, Right : Meters) return Meters
is abstract;