Strangest language feature

Question

What is, in your opinion, the most surprising, weird, strange or really "WTF" language feature you have encountered?

Please only one feature per answer.

Answer 1

PHP's handling of numeric values in strings. See this previous answer to a different question for full details but, in short:

"01a4" != "001a4"

If you have two strings that contain a different number of characters, they can’t be considered equal. The leading zeros are important because these are strings not numbers.

"01e4" == "001e4"

PHP doesn’t like strings. It’s looking for any excuse it can find to treat your values as numbers. Change the hexadecimal characters in those strings slightly and suddenly PHP decides that these aren’t strings any more, they are numbers in scientific notation (PHP doesn’t care that you used quotes) and they are equivalent because leading zeros are ignored for numbers. To reinforce this point you will find that PHP also evaluates "01e4" == "10000" as true because these are numbers with equivalent values. This is documented behaviour, it’s just not very sensible.

Answer 2

Not really a language feature, but an implementation flaw: Some early Fortran compilers implemented constants by using a constant pool. All parameters were passed by reference. If you called a function, e.g.

f(1)

The compiler would pass the address of the constant 1 in the constant pool to the function. If you assigned a value to the parameter in the function, you would change the value (in this case the value of 1) globally in the program. Caused some head scratching.

Answer 3

JavaScript truth table:

''        ==   '0'           // false
0         ==   ''            // true
0         ==   '0'           // true
false     ==   'false'       // false
false     ==   '0'           // true
false     ==   undefined     // false
false     ==   null          // false
null      ==   undefined     // true
" \t\r\n" ==   0             // true

Source: Doug Crockford

Answer 4

Trigraphs in C and C++.

int main() {
   printf("LOL??!");
}

This will print LOL|, because the trigraph ??! is converted to |.

Answer 5

The weird things C++ templates can be used for, best demonstrated by "Multi-Dimensional Analog Literals" which uses templates to compute the area of "drawn" shapes. The following code is valid C++ for a 3x3 rectangle

#include"analogliterals.hpp"
using namespace analog_literals::symbols;

          unsigned int c = ( o-----o
                             |     !
                             !     !
                             !     !
                             o-----o ).area;

Or, another example with a 3D cube:

  assert( ( o-------------o
            |L             \
            | L             \
            |  L             \
            |   o-------------o
            |   !             !
            !   !             !
            o   |             !
             L  |             !
              L |             !
               L|             !
                o-------------o ).volume == ( o-------------o
                                              |             !
                                              !             !
                                              !             !
                                              o-------------o ).area * int(I-------------I) );

Answer 6

INTERCAL is probably the best compendium of strangest language features. My personal favourite is the COMEFROM statement which is (almost) the opposite of GOTO.

COMEFROM is roughly the opposite of GOTO in that it can take the execution state from any arbitrary point in code to a COMEFROM statement. The point in code where the state transfer happens is usually given as a parameter to COMEFROM. Whether the transfer happens before or after the instruction at the specified transfer point depends on the language used. Depending on the language used, multiple COMEFROMs referencing the same departure point may be invalid, be non-deterministic, be executed in some sort of defined priority, or even induce parallel or otherwise concurrent execution as seen in Threaded Intercal. A simple example of a "COMEFROM x" statement is a label x (which does not need to be physically located anywhere near its corresponding COMEFROM) that acts as a "trap door". When code execution reaches the label, control gets passed to the statement following the COMEFROM. The effect of this is primarily to make debugging (and understanding the control flow of the program) extremely difficult, since there is no indication near the label that control will mysteriously jump to another point of the program.