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c++operator-precedence
Alex24
Alex24

Reputation: 610

Is postfix increment and decrement left to right associative or the contrary?

In "C++ primer 5th edition" on page 228 The table Table 4.4. Operator Precedence shows operators and associativitiy of operands.

What I am confused about is in this table says that Prefix increment/decrement is Right-To-Left associative and also Postfix increment/decrement also Right-To-Left So there's the letter "R" that means right to left. But in www.cppreference.com I see that postfix increment/decrement are Left-To-Right associative.

If someone makes things clear up through giving an example containing Compound expression, then is really appreciated.

Upvotes: 0

Views: 935

Answers (3)

Artur
Artur

Reputation: 7257

Prefix operators are Right to Left associative:

https://en.cppreference.com/w/cpp/language/operator_precedence

Neither C++ Primer 5th edition from Prata nor from Lippman have any operator priority table on page 228.

Upvotes: -1

Maestro
Maestro

Reputation: 2552

There's no book safe from Errata. Each version of a book adds some enhancements and corrects some mistakes. The author is always appreciating Errata reporting. Anyway: The post-fix increment and -decrement are Left-To-Right associative.

int x = 5;
x++;

As you can see from the expression above: the operand xis at the lhs of the operator ++ thus you easily can understand it.

++x;

Now the operand x is on the right thus Pre-increment/decrement is Right-To-Left.

Upvotes: 0

Pete Becker
Pete Becker

Reputation: 76305

The C++ grammar defines a postfix expression like this:

postfix-expression:
    primary-expression
    ...
    postfix-expression ++
    ....

In parsing a ++ ++, a is a primary-expression, so a ++ is a postfix-expression. The final ++ applies to the result of that postfix-expression.

In short, ++ groups left to right.

The same thing applies to all of what we usually think of as postfix operators: they apply to a postfix-expression, so they group left to right.

As mentioned in a comment, going the other way would make ptr[i]++ rather funky.

Looking at a (probably illegal) PDF version of that book that I found online, I suspect the the entries for postfix++ and postfix-- are cut-and-paste typos. Both of those operators are supposedly described on page 147, as are prefix++ and prefix--, but the only discussion there is about prefix++ and prefix--.

Upvotes: 5

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