PHP - 正确的三元实现[关闭]

Question

Ok,

so consider the following:

$this->foo = isset($_GET['foo']) && !empty($_GET['foo']) ? $_GET['foo'] : NULL;

and this:

$this->foo = (isset($_GET['foo']) && !empty($_GET['foo'])) ? $_GET['foo'] : NULL;

When I write an if / else statement with multiple checks, I generally include the extra parenthesis as in the second example. In the ternary, both examples work.

Should I add the extra parenthesis as on the bottom? Or go with the first?

Thanks

Answer 1

Operator precedence is the issue as to when parenthesis are necessary (notwithstanding readability).

When dealing with the ternary operator look at the order PHP uses to group expressions, start with the ternary operator(s) and examine the operators that are grouped after the ternary operator. Those are the operators that have the potential to produce erroneous output.

PHP Operator Precedence, starting with ternary:

Assoc.  Operators                   Additional Information
...
left    ? :                         ternary
right   = += -= *= **= /= .= %=  
        &= %= &= |= ^= <<= >>= =>   assignment
left    and                         logical
left    xor                         logical
left    or                          logical
left    ,                           many uses

In this case there are the assignment operators, the lower precedence logical operators, and comma.

It appears the ternary and assignment are equal, therefore grouping is determined by their associativity when the two are in the same statement.

$a = true? 'yes': 'no';
// $a is assigned 'yes'

Assignment is right associative so, in relation to the =, expressions are grouped right to left. In this case the ternary comes first (rightmost) and the statement works as expected.

That leaves the lower precedence boolean and the comma.

echo true and true? 'yes': 'no';
// Echos: 1
// Grouped like: echo true and (true? 'yes': 'no');

Not as expected. Use parenthesis to force intended grouping:

echo (true and true)? 'yes': 'no';
// Echos: yes

When using higher precedence boolean operators, which are grouped before the ternary operator, parenthesis are not necessary.

echo true && true? 'yes': 'no';
// Echos: yes

Bottom line, when operator precedence is unclear, or readability is desired, use parenthesis.