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Pre-increment (or pre-decrement) in C++

  • Difficulty Level : Medium
  • Last Updated : 12 Jul, 2018

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)




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

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




// CPP program to illustrate
// Post-increment (or post-decrement)
#include <cstdio>
  
int main()
{
    int a = 10;
    a++ = 20; // error
    printf("a = %d", a);
    getchar();
    return 0;
}
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 — 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 brevity, 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 which 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.

Want to learn from the best curated videos and practice problems, check out the C Foundation Course for Basic to Advanced C.



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