declaring a variable in if statement

Question

Please condiser the following snippet

//foo is just a random function which returns error code
if(int err = foo() == 0){
    //err should contain the error code
    ...
}

The problem with that is err contains the result of foo() == 0, and what I want to evalute is int err = foo(); then if(err == 0) but inside the if statement.

I have already tried if((int err = foo()) == 0) and it doesn't work.
Also, I don't want to do int err; if((err = foo) != 0)).

Is this possible ?

https://stackoverflow.com/questions/68967429/how-to-define-variable-and-compare-value-inside-if-statement (bad dupe target?) — user202729, Dec 13 '21 at 10:48
Not like this, but something similar works. `if (int err = foo())` is a boolean expression, which checks for 'Not 0', meaning you'll need to put your 'on Error' code into the else. — Refugnic Eternium, Dec 13 '21 at 10:48
If you have C++20, you can write `if(int err=foo(); err==0) {}` (or indeed 17 it seems). — DS_London, Dec 13 '21 at 10:50
You can but it doesn't always mean you should. Declaring it above the if statement improves readability. Your future self will thank you. — Offtkp, Dec 13 '21 at 20:37

score 1 · Accepted Answer · answered Dec 13 '21 at 10:50

1

For C++ 17 and onwards you can use if statements with initializers:

if (int err = foo(); err == 0) {
   ...
}

answered Dec 13 '21 at 10:50

Klickmann

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declaring a variable in if statement

1 Answers1