Comparison of ALGOL 68 and C++

C++ doesn't have:

ALGOL 68 doesn't have:

Comparison of the assignment and equality operators

Intent ALGOL 68 C++
Define a constant int x=888; const int x = 888;
Initialise a variable int x:=888; int x = 888;
Assign a value 888 to a variable x x:=888; x = 888;
Compare two values if x = 888 then ... fi if (x == 888) { ... }
Allocate a variable from the heap ref int x = heap int;
or simply:
heap int x; 		int* x = new int;
Compare address of two pointers ref int x, y;
if x :=: y then ... fi 		int* x; int* y;

if (x == y) { ... }

Compare value referenced by two pointers ref int x, y;
if x = y then ... fi 		int* x; int* y;

if (*x == *y) { ... }

Name a new type mode longreal = long real; typedef double longreal;
Name a new record type mode cust = struct(string name, address); struct cust { std::string name, address; };
Name a new union type mode taggedu = union(string s, real r); union u { std::string s; float f; };
Name a procedure or function proc f = (real x) real: ( code; result ); float f(float x) { code; return result; }
Procedure default parameters proc p = (union (real, void) in x)void:

    ( real x = (in x|(real x):x|888); code );

void p(float x=888) { code; }
Name a new operator op ↑ = (real x,y) real: x**y; N/A
Set priority on a new operator prio ↑ = 9; N/A
Chain variables assignment a:=b:=c:=d; a = b = c = d;
Displacement operator - ALGOL 68C only a:=:=b:=:=c:=:=d; a = b; b = c; c = d;
Append "substr" to a variable str str +:= "substr"; str += "substr";
Prefix "substr" to a variable str "substr" +=: str; str = "substr" + str;

Code Examples

Union declaration and use

Assigning values into an A68 union variable is automatic, the type is "tagged" to the variable, but pulling the value back out is syntactically awkward as a conformity-clause is required.

ALGOL 68 example:

 union(int, char) x:=666;
 printf(($3d l$, (x|(int i):i) ))

C/C++ example:

  union { int i; char c; } x = { 666 };
  std::cout << x.i << std::endl;

The net effect of "type-tagging" is that Algol68's strong typing "half" encroaches into the union.

Mode declaration

A new mode (type) may be declared using a mode declaration:

int max=99;
mode newtype = [0:9][0:max]struct (
   long real a, b, c, short int i, j, k, ref real r
);

This has the similar effect as the following C++ code:

const int max=99;
typedef struct { 
    double a, b, c; short i, j, k; float& r;
} newtype[9+1][max+1];

Note that for ALGOL 68 only the newtype name appears to the left of the equality, and most notably the construction is made - and can be read - from left to right without regard to priorities.

External links

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