# Pre-increment (or pre-decrement) With Reference to L-value in C++

Prerequisite: Pre-increment and post-increment in C/C++

In C++, pre-increment (or pre-decrement) can be used as l-value, but post-increment (or post-decrement) can not be used as l-value.
For example, following program prints a = 20 (++a is used as l-value)

l-value is simply nothing but the memory location, which has an address.

## CPP

 `// CPP program to illustrate` `// Pre-increment (or pre-decrement)` `#include `   `int` `main()` `{` `    ``int` `a = 10;`   `    ``++a = 20; ``// works` `    ``printf``(``"a = %d"``, a);` `    ``printf``(``"\n"``);` `    ``--a = 10;` `    ``printf``(``"a = %d"``, a);` `    ``return` `0;` `}`

Output:

```a = 20
a = 10```

Time Complexity: O(1)

The above program works whereas the following program fails in compilation with error “non-lvalue in assignment” (a++ is used as l-value)

## CPP

 `// CPP program to illustrate` `// Post-increment (or post-decrement)` `#include `   `int` `main()` `{` `    ``int` `a = 10;` `    ``a++ = 20; ``// error` `    ``printf``(``"a = %d"``, a);` `    ``return` `0;` `}`

Error:

```prog.cpp: In function 'int main()':
prog.cpp:6:5: error: lvalue required as left operand of assignment
a++ = 20; // error
^```

### How ++a is Different From a++ as lvalue?

It is because ++a returns an lvalue, which is basically a reference to the variable to which we can further assign â€” just like an ordinary variable. It could also be assigned to a reference as follows:

```int &ref = ++a; // valid
int &ref = a++; // invalid```

Whereas if you recall how a++ works, it doesn’t immediately increment the value it holds. For clarity, you can think of it as getting incremented in the next statement. So what basically happens is that, a++ returns an rvalue, which is basically just a value like the value of an expression that is not stored. You can think of a++ = 20; as follows after being processed:

```int a = 10;

// On compilation, a++ is replaced by the value of a which is an rvalue:
10 = 20; // Invalid

// Value of a is incremented
a = a + 1;```

That should help to understand why a++ = 20; won’t work. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Whether you're preparing for your first job interview or aiming to upskill in this ever-evolving tech landscape, GeeksforGeeks Courses are your key to success. We provide top-quality content at affordable prices, all geared towards accelerating your growth in a time-bound manner. Join the millions we've already empowered, and we're here to do the same for you. Don't miss out - check it out now!

Previous
Next