Why does ++a + ++a gives different answer in C++?

Question

So I tried this simple block
int a = 3;
cout << ++a + ++a;

I tried it in different compilers and I got 9 in some and 10 in the others. 9 seemed justified to me, but I could not wrap my head around why this gives 10. Please help.

Answer 1

I am not sure if they are using different precedence rule .. it does seem like UB im not completely sure why .....

ChillPill 🌶 sir ?

Clang gives 9
and gcc gives 10

for clang it works like so
(++a) + (++a) //9

for gcc its like this
(++(a++))+a //10

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

Clang ones seem like its wrong as postfix has more precedence....

Personal suggestion : never modify one variable twice in a single expression like this ....

Answer 2

I'm pretty sure that it is undefined behavior in the standard definition of the language, so the result is compiler-dependent.

Answer 3

It seems to depend on programming language, version/standard.

https://stackoverflow.com/questions/14798953/why-are-multiple-increments-decrements-valid-in-c-but-not-in-c

Answer 4

Not a direct answer to the question, but you might be interested in this thread as well:
https://www.sololearn.com/Discuss/2882645/?ref=app

Answer 5

In c/c++, when the statement have sequence point issue then it produce side effects and the output is compiler dependent , because it is undefined by standards..
https://en.m.wikipedia.org/wiki/Sequence_point

So it may evaluated as 4+5 or 5+5 or 5+4 Or
++(3+4) depends on compiler implementation..

edit:
additionally :
https://stackoverflow.com/questions/7721383/on-c-c-basically-what-things-are-compiler-dependent

Answer 6

Yeah a month ago I was also stuck with same.
Can you list me the compilers which gave you output 10.

Answer 7

Snehil Pandey SoloLearn also gives 10 and the interview bit and programiz compiler gives 10. The g++ that I use also gives 10 as output

Answer 8

rust wins again amiright ChillPill 🌶 ?

Answer 9

ChillPill 🌶 of course, this is not the place for such debates. i was merely joking.

Answer 10

ChillPill 🌶 looks like i passed the flags -Wrustacean -Wdefend ...

Answer 11

Prashanth Kumar you are invited to solve this issue

Answer 12

Prashanth Kumar Okay, thanks for clearing that out!

Answer 13

Depends on the compiler. Follow the one which follows fundamental concepts. Or you can put brackets in order to make it follow

Answer 14

a=3
++a=4
+
++a=5
=9
Different compilers may give different outputs but by programming logic, the answer should be 9.

Answer 15

(++a) + (++a) is much safer

Answer 16

The difference lies in which value is used in the rest of the expression containing it. With ++a, a is incremented first, and that is the value you get; with a++ you get the value of a, and a is incremented afterwards.