C++ Why we add parenthesis around assignments in if statements

05 January, 2026

Time to read: 1 mins

Last update: 05 January, 2026

I made a note for the question in the title back in 2022, but I always forget the answer.

From C++ Primer, 5th edition 2013, 4.4 “Assignment Operators” (p.146):

Because assignment (=) has lower precedence than the relational operators (<><=, etc.), parentheses are usually needed around assignments in conditions.

int i;
while ((i = get_value()) != 42) {
  // do something . . .
}

You can verify it with man pages:

$ apropos associativity
associativity: nothing appropriate.
$ apropos preced
operator (7)         - C operator precedence and order of evaluation
precedence (7)       - C operator precedence and order of evaluation
$ man 7 operator
...
NAME
       operator - C operator precedence and order of evaluation

DESCRIPTION
       This manual page lists C operators and their precedence in evaluation.

       Operator                            Associativity   Notes
       [] () . -> ++ --                    left to right   [1]
       ++ -- & * + - ~ ! sizeof            right to left   [2]
       (type)                              right to left
       * / %                               left to right
       + -                                 left to right
       << >>                               left to right
       < > <= >=                           left to right
       == !=                               left to right
       &                                   left to right
       ^                                   left to right
       |                                   left to right
       &&                                  left to right
       ||                                  left to right
       ?:                                  right to left
       = *= /= %= += -= <<= >>= &= ^= |=   right to left
       ,                                   left to right

Also an extra bit from C++ Primer p.145

The result of an assignment is its left-hand operand, which is an lvalue. The type of the result is the type of the left-hand operand. If the types of the left and right operands differ, the right-hand operand is converted to the type of the left.

C++ Why we add parenthesis around assignments in if statements

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