Chapter 9. Subprograms

Transcription

1 Chapter 9 Subprograms ISBN Chapter 9 Topics Introduction Fundamentals of Subprograms Design Issues for Subprograms Local Referencing Environments Parameter-Passing Methods Parameters That Are Subprogram Names Overloaded Subprograms Generic Subprograms Design Issues for Functions User-Defined Overloaded Operators Coroutines 9-2 1

2 Introduction Two fundamental abstraction facilities Process abstraction Emphasized from early days Data abstraction Emphasized in the1980s 9-3 Fundamentals of Subprograms Each subprogram has a single entry point The calling program is suspended during execution of the called subprogram Control always returns to the caller when the called subprogram s execution terminates 9-4 2

3 Basic Definitions (1/3) A subprogram definition describes the interface to and the actions of the subprogram abstraction A subprogram call is an explicit request that the subprogram be executed A subprogram header is the first part of the definition, including the name, the kind of subprogram, and the formal parameters 9-5 Basic Definitions (2/3) The parameter profile (aka signature) of a subprogram is the number, order, and types of its parameters The protocol is a subprogram s parameter profile and, if it is a function, its return type A subprogram declaration provides the protocol, but not the body, of the subprogram Function declarations in C and C++ are often called prototypes 9-6 3

4 Basic Definitions (3/3) A formal parameter is a dummy variable listed in the subprogram header and used in the subprogram An actual parameter represents a value or address used in the subprogram call statement caller d = f1(a, b, c) actual parameter formal parameter subprogram definition int f1(int x, float y, int z) { 9-7 Actual/Formal Parameter Correspondence (Positional) Positional The binding of actual parameters to formal parameters is by position the first actual parameter is bound to the first formal parameter and so forth Safe and effective caller d = f1(a, b, c) int f1(int x, float y, int z) { 9-8 4

5 Actual/Formal Parameter Correspondence (Keyword) Keyword The name of the formal parameter to which an actual parameter is to be bound is specified with the actual parameter Sumer (Length => My_Length, List => My_Array, Sum => My_Sum); In Ada Advantage: Parameters can appear in any order Disadvantage: user must know the formal parameter s names 9-9 Mixed Mode In Ada Positional Sumer (My_Length, Sum => My_Sum List => My_Array, ); Keyword

6 Formal Parameter Default Values In certain languages (e.g., C++, Ada), formal parameters can have default values (if no actual parameter is passed) definition function Compute_Pay(Income : Float; Exemptions : Integer := 1; Tax_Rate : Float) return Float; call Pay := Compute_Pay( , Tax_Rate => 0.15); then Exemptions has its default value Default Parameters in C++ In C++, default parameters must appear last because parameters are positionally associated definition call float compute_pay(float income, float tax_rate, int exemptions = 1) pay = compute_pay( , 0.15); then exemptions has its default value

7 Variable Number of Parameters in C# C# methods can accept a variable number of parameters as long as they are of the same type 9-13 Procedures and Functions There are two categories of subprograms Procedures are collection of statements that define parameterized computations Functions structurally resemble procedures but are semantically modeled on mathematical functions They are expected to produce no side effects In practice, program functions have side effects

8 Design Issues for Subprograms What parameter passing methods are provided? Are parameter types checked? Are local variables static or dynamic? Can subprogram definitions appear in other subprogram definitions? Can subprograms be overloaded? Can subprogram be generic? 9-15 Local Referencing Environments Local variables can be stack-dynamic (bound to storage) Advantages Support for recursion Storage for locals is shared among some subprograms Disadvantages Allocation/de-allocation, initialization time Indirect addressing Subprograms cannot be history sensitive

9 Local Referencing Environments Local variables can be static More efficient (no indirection) No run-time overhead Cannot support recursion #include <stdio.h> static int sum; stack-dynamic int fun1(int a, float b) { static int cnt; int i, j; static } stack-dynamic 9-17 Parameter Passing Methods Ways in which parameters are transmitted to and/or from called subprograms Pass-by-value Pass-by-result Pass-by-value-result Pass-by-reference Pass-by-name

10 Semantics Models of Parameter Passing In Mode Out Mode Inout Mode 9-19 Pass-by-Value (In Mode) The value of the actual parameter is used to initialize the corresponding formal parameter Normally implemented by copying Can be implemented by transmitting an access path but not recommended (enforcing write protection is not easy) When copies are used, additional storage is required Storage and copy operations can be costly

11 Illustrating Pass-by-Value caller d = f1(a, b, c) int f1(int x, float y, int z) { push(c) push(b) push(a) a b c stack x y z 9-21 Pass-by-Result (Out Mode) An implementation model for out-mode parameters the corresponding formal parameter acts as a local variable; its value is transmitted to caller s actual parameter when control is returned to the caller Require extra storage location and copy operation Potential problem: sub(p1, p1); whichever formal parameter is copied back will represent the current value of p

12 Pass-by-Result: C# Examples void DoIt(out int x, int index) { x = 17; index = 42; } sub = 21; f.doit(out list[sub], sub); If the address to which to return the value is computed on entry to the method, 17 will be returned to list[21] If the address is computed just before return, 17 will be returned to list[42] 9-23 Pass-by-Value-Result (Inout Mode) A combination of pass-by-value and pass-by-result Sometimes called pass-by-copy Formal parameters have local storage Disadvantages: Those of pass-by-result Those of pass-by-value

13 Pass-by-Reference (for Inout Mode) Pass an access path (usually an address) Also called pass-by-sharing Passing process is efficient (no copying and no duplicated storage) Disadvantages Slower accesses (compared to pass-by-value) to formal parameters Potentials for un-wanted side effects Un-wanted aliases (access broadened) 9-25 Creating Alias With Pass-By-Reference definition void fun(int &first, int &second) call fun(total, total) call (if i == j) fun(list[i], list[j])

14 Example of Creating Alias in C int * global; void main() { sub(global); } void sub(int * param) { } global and param are aliases 9-27 Pass-by-Name (Inout Mode) By textual substitution Formals are bound to an access method at the time of the call, but actual binding to a value or address takes place at the time of a reference or assignment Allows flexibility in late binding

15 Pass-by-Name: Resulting Semantics If actual is a scalar variable, it is pass-by-reference If actual is a constant expression, it is pass-byvalue If actual is an array element, it is like nothing else procedure sub1(x: int; y: int); begin x := 1; y := 2; x := 2; y := 3; end; sub1(i, a[i]); 補 充 教 材 9-29 Pass-by-Name: Resulting Semantics If actual is an expression with a reference to a variable that is also accessible in the program, it is also like nothing else e.g. (assume k is a global variable) procedure sub1(x: int; y: int; z: int); begin k := 1; y := x; k := 5; z := x; end; sub1(k+1, j, i); 補 充 教 材

16 Implementing Parameter-Passing Methods In most language parameter communication takes place through the run-time stack Pass-by-value parameters have their values copied into stack locations, which then serve as storage for corresponding formal parameters Pass-by-result parameters are implemented as the opposite of pass-by-value Pass-by-reference is the simplest to implement; only an address is placed in the stack 9-31 One Possible Stack Implementation Passed by value Passed by result Passed by value-result Passed by reference

17 Parameter Passing Methods of Major Languages (Fortran and C) Fortran C Always used the inout semantics model Before Fortran 77: pass-by-reference Fortran 77 and later: scalar variables are often passed by value-result Pass-by-value Pass-by-reference is achieved by using pointers as parameters 9-33 Parameter Passing Methods of Major Languages (C++ and Java) C++ A special pointer type called reference type for pass-by-reference Java All parameters are passed by value Object parameters are passed by reference

18 Parameter Passing Methods of Major Languages (Ada) Ada Three semantics modes of parameter transmission: in, out, in out; in is the default mode Formal parameters declared out can be assigned but not referenced those declared in can be referenced but not assigned in out parameters can be referenced and assigned 9-35 Parameter Passing Methods of Major Languages (C# and Others) C# Default method: pass-by-value Pass-by-reference is specified by preceding both a formal parameter and its actual parameter with ref PHP: very similar to C# Perl: all actual parameters are implicitly placed in a predefined array

19 Type Checking Parameters Considered very important for reliability FORTRAN 77 and original C: none Pascal, FORTRAN 90, Java, and Ada: it is always required ANSI C and C++: choice is made by the user Prototypes Relatively new languages Perl, JavaScript, and PHP do not require type checking 9-37 In C89, functions can be defined in the original C double sin(x) double x; { } avoid type checking with prototype method double sin(double x) { } type checking performed double value; int count; value = sin(count) legal but incorrect double value; int count; value = sin(count) legal yet correct

20 In C99 and C++ All functions must have their formal parameters in prototype format Type checking can be avoided for some of the parameters By replacing the last part of the parameter list with an ellipsis int printf(cost char* format_string, ) 9-39 Multidimensional Arrays as Parameters If a multidimensional array is passed to a subprogram and the subprogram is separately compiled, the compiler needs to know the declared size of that array to build the storage mapping function void fun(int matrix[][]) { a = matrix[i][j] + 1; } Where is matrix[i][j]?

21 Multidimensional Arrays as Parameters: C and C++ Programmer is required to include the declared sizes of all but the first subscript in the actual parameter void fun(int matrix[][10]) { } void main() { int mat[5][10]; fun(mat); } for the storage mapping function Note: C array is row-major 9-41 Multidimensional Arrays as Parameters: C and C++ (Continued) Disallows writing flexible subprograms Programmer s solution: pass a pointer to the array and the sizes of the dimensions as other parameters void fun(float *mat_ptr, int num_rows, int num_cols); the user must include the storage mapping function in terms of the size parameters *(mat_ptr +(row * num_cols) + col) = x; (programmer 自 己 寫 的 )

22 Multidimensional Arrays as Parameters: Ada Constrained arrays: Not a problem; declared size is part of the array s type Unconstrained arrays index ranges are not given in the array type definition. can be formal parameters cause problems? type Mat_Type is array (Integer range <>, Integer range <>) of Float; unconstrained array type function Sumer(Mat : in Mat_Type) return Float 9-43 Ada s Solution Definitions of variables of unconstrained array types must include index ranges. Subprogram can obtain the index range information of the actual parameter type Mat_Type is array (Integer range <>, Integer range <>) of Float; Mat_1 : Mat_Type(1..100, 1..20); index range function Sumer(Mat : in Mat_Type) return Float is begin for Row in Mat range(1) loop for Col in Mat range(2) loop get ranges

23 Multidimensional Arrays as Parameters: Fortran Formal parameter that are arrays have a declaration after the header For single-dimension arrays, subscripts in the declaration are irrelevant For multi-dimensional arrays, the subscripts allow the compiler to build the storage-mapping function 9-45 Fortran Example Subroutine Sub(Matrix, Rows, Cols, Result) 傳 入 參 數 Integer, Intent(In) :: Rows, Cols Real, Dimension(Rows, Cols), Intent(In) :: Matrix Real, Intent(In) :: Result 陣 列 的 大 小 End Subroutine Sub array declaration

24 Multidimensional Arrays as Parameters: Java and C# Similar to Ada Arrays are objects; they are all singledimensioned, but the elements can be arrays Each array inherits a named constant (length in Java, Length in C#) that is set to the length of the array when the array object is created 9-47 Example in Java row 的 大 小 float sumer(float mat[ ][ ]) { float sum = 0.0f; for (int row = 0; row < mat.length, row++) { for (int col = 0; col < mat[row].length; col++ { sum += mat[row][col]; } } column 的 大 小 return sum; }

25 Design Considerations for Parameter Passing Two important considerations Efficiency One-way or two-way data transfer But the above considerations are in conflict Good programming suggest limited access to variables, which means one-way whenever possible But pass-by-reference is more efficient to pass structures of significant size (like an array) 請 自 行 閱 讀 9-49 Parameters that are Subprogram Names It is sometimes convenient to pass subprogram names as parameters Issues: 1. Are parameter types (of the passed subprogram) checked? 2. What is the correct referencing environment for a subprogram that was sent as a parameter? (the problem arises only with languages that allow nested subprograms)

26 Parameters that are Subprogram Names: Parameter Type Checking C and C++ functions cannot be passed as parameters, but pointers to functions can be passed The type of the pointer is the function s protocol So parameters can be type checked accepting a pointer to function int fun1(int x, float y); int (*p)(int,float); p = fun1; fun2(p); p is a pointer to function int fun2(int (*fc)(int, float)) { fc(7, 12.3); return 0; } passing p as a parameter 9-51 Parameters that are Subprogram Names: Parameter Type Checking FORTRAN 95 type checks Ada does not allow subprogram parameters; a similar alternative is provided via Ada s generic facility

27 Parameters that are Subprogram Names: Referencing Environment What is the referencing environment of the subprogram that is passed as a parameter? Shallow binding: The environment of the call statement that enacts the passed subprogram Deep binding: The environment of the definition of the passed subprogram Ad hoc binding: The environment of the call statement that passed the subprogram 9-53 Referencing Environment Example function sub1() { var x; function sub2() { alert(x); }; function sub3() { var x; x = 3; sub4(sub2); }; function sub4(subx) { var x; x = 4; subx(); }; x = 1; sub3(); }; sub2 中 的 x 是 哪 裏 的? Shallow binding: sub2 中 的 x 是 sub4 的 Deep binding: sub2 中 的 x 是 sub1 的 Ad hoc binding: sub2 中 的 x 是 sub3 的

28 Overloaded Subprograms An overloaded subprogram is one that has the same name as another subprogram in the same referencing environment Every version of an overloaded subprogram has a unique protocol double pow(double, double) double pow(int, double) double pow(float, double) overloaded C++, Java, C#, and Ada include predefined overloaded subprograms 9-55 Overloaded Subprograms (Continued) In Ada, the return type of an overloaded function can be used to disambiguate calls (thus two overloaded functions can have the same parameter profile and differ only in their return types) function f1(p1: in Integer) return Float; function f1(p1: in Integer) return Integer; Because Java, C++, and C# allow mixed-mode expressions, the return type is irrelevant to the disambiguation ( 因 為 有 coercion) int fun(int p1); float fun(int p1); Which one should be called in fun(3)?

29 Generic Subprograms A generic or polymorphic subprogram takes parameters of different types on different activations Overloaded subprograms provide ad hoc polymorphism (they need not behave similarly) parametric polymorphism is provided by a subprogram that takes generic parameters that are used in type expressions that describes the types of the parameters of the subprogram 9-57 Examples of parametric polymorphism: C++ template <class Type> Type max(type first, Type second) { } return first > second? first : second; The above template can be instantiated for any type for which operator > is defined. For example int max (int first, int second) { return first > second? first : second; } 只 寫 一 次 程 式 碼, 產 生 對 應 不 同 參 數 型 態 的 版 本

30 Design Issues for Functions Are side effects allowed? Parameters should always be in-mode to reduce side effect (like Ada) What types of return values are allowed? Most imperative languages restrict the return types C allows any type except arrays and functions C++ is like C but also allows user-defined types Ada allows any type Java and C# do not have functions but methods can have any type 9-59 User-Defined Overloaded Operators Operators can be overloaded in Ada and C++ An Ada example Function * (A,B: in Vec_Type): return Integer is Sum: Integer := 0; begin for Index in A range loop Sum := Sum + A(Index) * B(Index) end loop return sum; end * ; 將 * 定 義 成 內 積 運 算 c = a * b; -- a and b are of type Vec_Type

31 Coroutines A coroutine is a subprogram that has multiple entries and controls them itself Also called symmetric control: caller and called coroutines are on a more equal basis A coroutine call is named a resume The first resume of a coroutine is to its beginning, but subsequent calls enter at the point just after the last executed statement in the coroutine Coroutines repeatedly resume each other, possibly forever Coroutines provide quasi-concurrent execution of program units (the coroutines); their execution is interleaved, but not overlapped 9-61 Coroutines Illustrated: Possible Execution Controls

32 Coroutines Illustrated: Possible Execution Controls 9-63 Coroutines Illustrated: Possible Execution Controls with Loops

33 Summary A subprogram definition describes the actions represented by the subprogram Subprograms can be either functions or procedures Local variables in subprograms can be stackdynamic or static Three models of parameter passing: in mode, out mode, and inout mode Some languages allow operator overloading Subprograms can be generic A coroutine is a special subprogram with multiple entries