Difference between Foreach and For Loops?

Question

What's the real difference between a foreach and for loop if either can get the same job done? I'm learning C++ and apparently there is no foreach loop for its arrays :(

Answer 1

There is no "foreach" language construct in C++, a least not literally. C++11 introduces something that's "as good as" a foreach loop, though.

The traditional for loop has something to do with evaluating conditions and performing repeated operations. It's a very general control structure. Its most popular use is to iterate over container or array contents, but that's just a tiny fraction of what you can do with it.

A "foreach" loop, on the other hand, is explicitly designed to iterate over container elements.

Example:

int arr[5] = { 1, 3, 5, 2, 4 };

for (int & n : arr) { n *= 2; } // "for-each" loop, new in C++11

for (size_t i = 0; i != 5; ++i) { arr[i] *= 2; } // "classic" for loop

In the second for, we use a traditional for loop to increment an auxiliary variable i in order to access the container arr. The first, range-based loop does not expose any details of the iteration, but just says "do this and that to each element in the collection".

Since the traditional for loop is a very general control structure, it can also be used in unusual ways:

std::vector<std::string> all_lines;
for (std::string line; std::cin >> line; all_lines.push_back(line))
{
  std::cout << "On line " << (all_lines.size() + 1) << " you said: " << line << std::endl;
}

You can trivially rewrite for(A; B; C) as a while loop:

{  // scope!
  A;
  while (true && B)
  {
    {  // more scope!
      /* for loop body */
    }
    C;
  }
}

Edit: I would probably be remiss not to mention the library function template std::for_each from <algorithm>, which in conjunction with lambdas is a very nice and self-descriptive way to iterate over arbitrary ranges (not just entire containers). It has existed since Day 1, but before lambdas it was a show-stopping pain to use.

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Update: I thought of something else that might be relevant here: A "foreach" loop generally assumes that you don't modify the container. A common type of looping that modifies the container requires the traditional for-loop; as for example in this typical erase pattern:

for(Container::const_iterator it = v.begin(); it != v.end() /* not hoisted! */; /* no increment */ )
{
  // do something
  if (suitable_condition)
  {
    v.erase(it++);   // or it = v.erase(it), depending on container type
  }
  else
  {
    ++it;
  }
}