When do we use i++ and when do we use ++i in C++

Question

When I do exercises, I always use for loops: for(int i = 0; i < n; i++) But sometimes I got wrong with them, I have tried in many ways to fix, but it ran only when I turn the i++ in for loop to ++i. I wonder when can we use i++ and when ++i?

Answer 1

i++ and ++i are statements and expressions at the same time.

They both increment the i variable, obviously, and both return the value of i as their result, but i++ returns the value of i before it's incremented whereas ++i returns the value of i after it's incremented.

If you don't use it as an expression but just as a statement, either form works equally well. If you do use the result, it depends what you want to achieve, e.g. in a[i++] = 1 you write to a and then increment the index.

Answer 2

++i is called pre increment while i++ is called postincrement.

++i increments the value of i and then returns i,
i++ returns the value of i and then increments.

Both of them increase the variable i by one. It's like saying i = i + 1. The difference is subtle. If you're using it in a loop like this, there's no difference:

for (int i = 0; i < 10; i++) {
}

for (int i = 0; i < 10; ++i) {
}

Answer 3

Two typical implementations where T represents any type:

++i prefix increment

T& operator++() {   // returns by reference
    // increment *this by 1 here
    return *this;
}

i++ postfix increment

T operator++(int) { // returns by value
    T copy(*this);  // make a copy of the current state
    ++(*this);      // increment *this by 1 here, using the pre-increment operator
    return copy;    // return the copy
}