jbode

 Profile Answers by jbode Questions by jbode

• Jul 11th, 2017

 

The main differences between `b++` and `b = b + 1` is that in the first case, `b` is only evaluated once, and the result of the first expression is the value of `b` *before* the increment.
`b++` evaluates to the current value of `b`, and as a *side effect* increments `b`. `++b` evaluates to the current value of `b` plus 1, and as a side effect increments `b`. See code snippet for examples.
In an expression like `a = b++`, its not guaranteed that `b` is incremented before `a` is assigned; the operations are *unsequenced* relative to each other. C does not force left-to-right evaluation in most cases, nor does it require that the side effect of the `++` operator be applied immediately after evaluation. This means that expressions of the form `b++ * b++` or `a[i] = i++` or similar invoke *undefined behavior* - youre not guaranteed of getting the result you expect.

Code
int main( void )


{


  int a, b = 0;


 


  a = b++; // a == 0, b == 1


  b = 0;


  a = ++b; // a == 1, b == 1


 


  b = 0;


  a = b = b + 1; // == a = (b = b + 1), a == 1, b == 1


}