Ada language - How to store a string value returned by a function?

Question

I'm attempting to some basic command-line interaction in Ada 2012 and I can't find a way to capture the string returned from the Ada.Command_Line.Command_Name() function.

All the examples I can find online simply print the String using Put() without first storing it in a local variable. Here's the faulty code I've tried, which does compile but throws a CONSTRAINT_ERROR ... length check failed when I try to assign the return String value to the String variable...

with Ada.Command_Line; use Ada.Command_Line;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Strings; use Ada.Strings;

procedure Command_Line_Test is
  ArgC : Natural := 0;
  InvocationName : String (1 .. 80);
begin
  ArgC := Argument_Count;
  Put ("Number of arguments provided: ");
  Put (ArgC'Image);
  New_Line;

  InvocationName := Command_Name;  --  CONSTRAINT_ERROR here
  Put ("Name that the executable was invoked by: ");
  Put (InvocationName);
  New_Line;

end Command_Line_Test;

I'm just using Command_Name as an example, but imagine it was any other function that could return a string (perhaps a string that would change multiple times during the program's life), how should we declare the local variable in order to store the returned string?

Answer 1

String handling in Ada is very different from other programming languages and when you declare a String(1..80) it expect that string returned by a function will actually have length 1..80 while your executable path (which is returned by Command_Name) may be a bit shorter (or longer).

You may always introduce new declare block and create a String variable inside of it like here

with Ada.Command_Line; use Ada.Command_Line;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Strings; use Ada.Strings;

procedure Main is
   ArgC : Natural := 0;

begin
   ArgC := Argument_Count;
   Put ("Number of arguments provided: ");
   Put (ArgC'Image);
   New_Line;
   declare
      InvocationName: String := Command_Name;  --  no more CONSTRAINT_ERROR here
   begin
      Put ("Name that the executable was invoked by: ");
      Put (InvocationName);
      New_Line;
   end;

end Main;

Or you may use Ada.Strings.Unbounded package

with Ada.Command_Line; use Ada.Command_Line;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Strings; use Ada.Strings;
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;

procedure Main is
   ArgC : Natural := 0;
   InvocationName : Unbounded_String;
begin
   ArgC := Argument_Count;
   Put ("Number of arguments provided: ");
   Put (ArgC'Image);
   New_Line;

   InvocationName := To_Unbounded_String(Command_Name);  --  no more CONSTRAINT_ERROR here

   Put ("Name that the executable was invoked by: ");
   Put (To_String(InvocationName));
   New_Line;

end Main;

Answer 2

Agree with Timur except that there is no need to move the declaration of Invocation_Name into a nested declare block.

You could have just written;

   Invocation_Name : String := Command;

Where it was in the original code, in the declarations of procedure Main.

or better yet;

   Invocation_Name : constant String := Command;

or better yet, eliminate the declaration altogether and replace the last 3 lines of Main with;

   Put_Line ("Name that the executable was invoked by: " & Command);