Pre Increment is not returning l-value

Question

// Pre-increment (or pre-decrement) 
#include <cstdio> 

int main() 
{ 
    int a = 10; 

    ++a = 20; // works 

    //printf("a = %d", ((++a)++)); 
    getchar(); 
    return 0; 
}

It is given in this article that ++a=20 works but when I am running on ideone, this code is giving an error.

https://www.geeksforgeeks.org/g-fact-40/

https://ideone.com/12DmS7

What are you trying to accomplish by doing `++a = 20`? Do you want 20, or 21? — Frontear, Apr 25 '19 at 13:12
*but when I am running on ideone, this code is giving an error.* – [C != C++ and C++ != C](https://ideone.com/1aZnHd). The article you link to is about C++. But you compile your code as C in ideone. — Swordfish, Apr 25 '19 at 13:13
@John3136, the geeksforgeeks article, I have mentioned the link. — Dharmender Lodhi, Apr 25 '19 at 13:14
And what is the error message? Can you please copy it into the question!? — Werner Henze, Apr 25 '19 at 13:17
Regardless of what language you use, you should never write an expression that modifies same variable twice. — user694733, Apr 25 '19 at 13:18
not the answer: Nobody should write such code in the first place. You either want `a = 20;` or `a = 21;` so simply write that. Writing `++a = 20;` is fine for articles for geeks or to ponder about some rules of the language, but it should never pass a serious code review — 463035818_is_not_an_ai, Apr 25 '19 at 13:27
Possible duplicate of [The difference between C and C++ regarding the ++ operator](https://stackoverflow.com/questions/25654339/the-difference-between-c-and-c-regarding-the-operator) — L. F., Apr 25 '19 at 14:05
I'm reminded of the initialism WYGIWYD: What You Get Is What You Deserve. — user888379, Apr 25 '19 at 15:07

score 4 · Accepted Answer · answered Apr 25 '19 at 13:28

4

This is one difference between C and C++.

In C, ++a is not an l-value, so it can't be on the left hand side of an assignment.

In C++ it can be.

Some compilers that purportedly compile C code (e.g. MSVC) emit the construct in error.

answered Apr 25 '19 at 13:28

Bathsheba

231,907
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score 1 · Answer 2 · edited Jun 20 '20 at 09:12

1

This code works well in ideone too(See the screenshot below). Run it using C++ not C. Pre-increment works in C++ but it gives a compilation error in C.

edited Jun 20 '20 at 09:12

Community

1
1

answered Apr 25 '19 at 13:19

Niloy Rashid

687
4
16

Pre Increment is not returning l-value

2 Answers2