Declare variable in if statement (ANSI C)

Question

Is there any way to declare variable in if statement (using ANSI C only) ?

Example:

if(int variable = some_function())
{
    return 1;
}

http://stackoverflow.com/questions/8474100/where-you-can-and-cannot-declare-new-variables-in-c — Jorge Cavaiuolo, Apr 26 '17 at 19:56
According to the [ANSI C Grammar](https://www.lysator.liu.se/c/ANSI-C-grammar-y.html#expression), the grammar for the if statement is `IF '(' expression ')' statement`. `expression` cannot resolve to `declaration`, so there is no way to put a declaration in an if statement like in your example. — Random Davis, Apr 26 '17 at 19:58
Declare a variable strictly using a classic declaration of format `type var = value`? No. — sjsam, Apr 26 '17 at 20:05
@RandomDavis That grammar is not up to date (but that particular production hasn't changed). Note that `IF` in that grammar refers to the `if` keyword (`IF` is an ordinary identifier). The grammar in the ISO C standard is **`if`** **`(`** *expression* **`)`** *statement* or **`if`** **`(`** *expression* **`)`** *statement* **`else`** *statement* — Keith Thompson, May 01 '20 at 03:21
I never used to appreciate the value of this construct until I realized how repetitive code gets when you are checking an error flag. — Sridhar Sarnobat, Mar 09 '22 at 05:12

pmg · Accepted Answer · 2022-03-09T08:19:13.373

8

No, you cannot do that.

What you can do is create a compound statement (anonymous or hanging block) just for the if

    {
        int variable;
        variable = some_function();
        if (variable) return 1;
    }
    /* variable is out of scope here */

Note that for this simple case you can call the function as the condition of the if (no need for an extra variable)

if (some_function()) return 1;

edited Mar 09 '22 at 08:19

answered Apr 26 '17 at 19:55

pmg

If you mean anonymous braces, I'm happy about that. I think they're underrated and are a great first step for refactoring. – Sridhar Sarnobat Mar 09 '22 at 05:03
1

The Standard calls them "compound statements" ([C11 6.8.2](http://port70.net/~nsz/c/c11/n1570.html#6.8.2)) – pmg Mar 09 '22 at 08:18

1 Answers1