Safe modern for loops

I had the opportunity to accompany many students while they were learning C. Looking at their coding style, I noticed some bad habits that could lead to hard-to-detect bugs in real-world problems. I’ll show the tools that Modern C++ brings that can prevent that kind of problems.

Pre-C++11 for loops

Before discussing the problems themselves, let’s review how for loops work in C++.

In a very basic way, the code

for (/* init_statement */; /* condition */; /* iteration_expression */) {
  /* statement */
}

is translated to

{
  /* init_statement */;
  while ( /* condition */ ) {
    /* statement */
    /* iteration_expression */;
  }
}

Thus, we can call an arbitrary function do_something for each integer between 0 (inclusive) and 10 (non-inclusive) by using

for (int i = 0; i < 10; ++i)
  do_something(i);

We can also use for loops to iterate over containers.

std::vector<int> values( /* ... */ );

for (typename std::vector<int>::iterator it = values.begin();
     it != values.end();
     ++it)
{
  do_something(*it);
}

For a better review, I suggest reading this page.

C++11 range-based for loops

Modern C++ revisions (after C++11) bring a much better way to iterate over containers: the range-based for loops.

The last piece of code is equivalent to

std::vector<int> values( /* ... */ );

for (int i : values)
  do_something(i);

For a better review, I suggest reading this page.

Problem: Value range

Now, let’s discuss the bad habits that I have seen. Imagine we want to iterate over a vector and call a user-defined function that receives both the index and the value at that position. A faulty solution would be

template <typename T, typename F>
void my_solution(const std::vector<T>& values, const F& f) {
  for (int i = 0; i < values.size(); ++i)
    f(i, values[i]);
}

Can you spot the bug? Many of us are so used to iterate using ints that we forget that it can’t represent all possible index values. The problem lays in the fact that most likely

std::numeric_limits<int>::max() < std::numeric_limits<typename std::vector<T>::size_type>::max()

std::vector<T>::size_type is the type that can hold any index or size of vectors. Although many implementations use std::size_t, there is no such guarantee. Thus, a better generic solution is

template <typename T, typename F>
void my_solution(const std::vector<T>& values, const F& f) {
  for (typename std::vector<T>::size_type i = 0; i < values.size(); ++i)
    f(i, values[i]);
}

Problem: Iterating two containers

Another problem that can happen is regarding the const-correctness of the indices. It doesn’t come exactly from a bad habit, but from a limitation in the traditional for loop.

Consider the same situation as before, but now we want to have a callback for every combination of values in two containers. Based on what we have discussed, a common mistake is

template <typename T, typename F>
void my_solution(const std::vector<T>& a, const std::vector<T>& b, const F& f) {
  for (typename std::vector<T>::size_type i = 0; i < a.size(); ++i)
    for (typename std::vector<T>::size_type j = 0; j < b.size(); ++i)
      f(i, j, a[i], b[j]);
}

In the second for loop, instead of incrementing j we increment i causing a bug that is often hard to detect. A possible cause of the problem here is that variables have no meaningful name. However, it is so common to iterate using variables called i and j that long meaningful names would be awkward.

An ideal solution would forbid modifications of the iteration variable inside the for loop. Thus, if we tried to modify the variable i in the second loop, a compiler error would be emitted. However, just using const is not enough. The code

template <typename T, typename F>
void my_solution(const std::vector<T>& a, const std::vector<T>& b, const F& f) {
  for (const typename std::vector<T>::size_type i = 0; i < a.size(); ++i)
    for (const typename std::vector<T>::size_type j = 0; j < b.size(); ++j)
      f(i, j, a[i], b[j]);
}

doesn’t compile since ++i and ++j tries to modify the variables.

Solution: cool::indices utility

By using range-based for loops, cool provides a utility that solves the two mentioned problems. The function cool::indices(n, m) creates a lazy-evaluated list of indices in the interval whose type is big enough to hold m and n (or their own type if they have the same type.) If only one value is provided, that is cool::indices(m) is called, the range goes from 0 (inclusive) to n (exclusive.)

That brings the solution

template <typename T, typename F>
void my_solution(const std::vector<T>& a, const std::vector<T>& b, const F& f) {
  for (const auto i : cool::indices(a.size()))
    for (const auto j : cool::indices(b.size()))
      f(i, j, a[i], b[j]);
}

which has several advantages:

  • i and j have type std::vector<T>::size_type without explicitly writing so;
  • the compiler would emit an error if one tries to modify i and j; and
  • there are much fewer occurrences of the variables (no explicit comparison and increment), reducing the chances of mistyping.
Written on October 11, 2017