Reading from Iterator yields new value before it gets incremented in next argument

Question

I'm trying to sum up the distances of a closed path in some graph. The path, stored as a vector of ints representing the nodes, begins with 0 and is supposed to have another edge back from the last node to 0. So I'm doing

using namespace std;
typedef vector<int> Route;

// r is some Route, g is some Graph
int distance = 0;
for (Route::iterator it = r.begin(); it != r.end(); ) {
    clog << "it: " << *it << endl;
    distance += g.dist(*it, (++it == r.end()) ? 0 : *it);
}

However, the first argument of my dist method is receiving the wrong value. The logging in the line above does output the correct value, but logging the arguments from dist shows that it does get passed the same value twice.

In the last iteration, it does get passed 0 which is plain wrong. Sometimes, with bigger vectors (I've got another input data with 120 waypoints), the first argument becomes some abstruse large value (which breaks the distance function) while the second argument is 0 (as supposed).

What does happen here and why does it happen? I have solved this now with a temporary variable for the first argument, but it seems ugly and unnecessary.

---

For completeness, here's the code of my dist method which is part of a Graph class (basically an adjacency matrix):

int dist(size_t x, size_t y) {
    clog<<"distance: "<<x<<" "<<y<<endl;
    if (x == y) return 0;
    if (x < y) return dist(y, x); // edges are symmetric
    if (x > size)
        throw out_of_range("Graph::dist: index too large");
    return triangle[x*(x-1)/2+y]; // stored in a flat array representing the lower half of the matrix
}

Here's the output for a small example (vector [0, 1, 2, 3]):

it: 0
distance: 1 1
it: 1
distance: 2 2
it: 2
distance: 3 3
it: 3
distance: 0 0
          ^^^ the distance arguments are wrong

and for the large example I'm getting

[…]
distance: 32 32
it: 32
distance: 51 51
it: 51
distance: 90 90
it: 90
distance: 12 12
it: 12
distance: 859381811 0
          ^^^^^^^^^ WTH?
terminate called after throwing an instance of 'std::out_of_range'
  what():  Graph::dist: index too large

I had expected to get (for the small example):

it: 0
distance: 0 1
it: 1
distance: 1 2
it: 2
distance: 2 3
it: 3
distance: 3 0

Answer 1

The compiler made an optimization when you are using *it in the function call. The following works:

#include <iostream>
#include <vector>
using namespace std;

int dist(size_t x, size_t y) {
    cout<<"distance: "<<x<<" "<<y<<endl;
    return y - x;
}

int main() {
  // r is some Route, g is some Graph
  vector<int> r = {1, 2, 3, 4};
  int distance = 0;
  for (auto it = r.begin(); it != r.end(); ) {
      cout << "it: " << *it << endl;
      int prev = *it;
      ++it;    
      int current = (it == r.end()) ? 0 : *it;
      distance += dist(prev, current);
  }
}

I am still digging the standard and trying to figure out why that optimization is allowed. Evaluation order doesn't really explain it.

========== Edit ===========================

I was wrong. Evaluation order is the cause of the problem. In this case, your ternary operator was evaluated in the correct order, but gcc is also free to move around the subexpression of the ternary operator. The assembly of your code looks like the following:

  movq  %rax, %rdi  # tmp96,
  call  _ZN9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEppEv #
  leaq  -32(%rbp), %rdx #, tmp97
  movq  %rdx, %rsi  # tmp97,
  movq  %rax, %rdi  # D.33388,
  call  _ZN9__gnu_cxxeqIPiSt6vectorIiSaIiEEEEbRKNS_17__normal_iteratorIT_T0_EESA_ #

      # If the test succeeds, go to fetch the content of the iterator
      # otherwise just set it to zero and proceed.
  testb %al, %al  # D.33389
  je  .L5 #,
  movl  $0, %ebx  #, iftmp.2
  jmp .L6 #
.L5:
  leaq  -64(%rbp), %rax #, tmp98
  movq  %rax, %rdi  # tmp98,

      # fetch the content of the iterator if we need it
  call  _ZNK9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEdeEv  #
  movl  (%rax), %eax  # *D.33393_16, D.33394
  movslq  %eax, %rbx  # D.33394, iftmp.2
.L6:
  leaq  -64(%rbp), %rax #, tmp99
  movq  %rax, %rdi  # tmp99,

      # deference the iterator again. NOTE iterator here was NOT INTCREMENTED
  call  _ZNK9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEdeEv  #
  movl  (%rax), %eax  # *D.33395_19, D.33396
  cltq
  movq  %rbx, %rsi  # iftmp.2,
  movq  %rax, %rdi  # D.33397,
  call  _Z4distmm #
  addl  %eax, -24(%rbp) # D.33398, distance

Pay attention to the evaluation of *it. In this case, the iterator was incremented if necessary, the ternary operator condition is evaluated, and then it is dereferenced.

============ Edit ===========

The order of evaluation in C++ for function parameters and its subexpressions are undefined. More information can be found on function parameter evaluation order. There is also a really nice artcle on the details of evaluation order written on cppreference