Writing backwards

We want to write a function that reverse the words in input. Just the order of words is reverted, not their actual structure. If we get in input "hello" and "world" we want in output "world hello" and not "dlrow olleh". Notice that there is a blank between each couple of words, but not at the end of the sentence.

This problem is based on the CodeEval Reverse world challenge.

A few test cases (C++11 for GTest as xUnit framework) would clarify what I'd expect from that function:

TEST(ReWo, Given) // 1
{
  ASSERT_EQ(solution( { "Hello", "World" } ), "World Hello");
}

TEST(ReWo, OneWord) // 2
{
  ASSERT_EQ(solution( { "Hello" } ), "Hello");
}

TEST(ReWo, Empty) // 3
{
  ASSERT_DEATH(solution( { } ),"Assertion .* failed.");
}

1. Vanilla case, the words in input are combined in a string from the last one up.
2. A single word in input should not break the algorithm.
3. It is a caller responsibility to ensure that at least a word is passed. To make this requirement explicit in the code, I want my function to assert the input being not empty. I could have been less strict, and simply return an empty string, but I wanted to show how to use the GoogleTest ASSERT_DEATH macro. More on the same topic on a previous post.

Here is how implemented in plain C++98 the function:

std::string solution(const std::vector<std::string>& input)
{
  assert(!input.empty()); // 1

  std::ostringstream oss; // 2
  for(std::vector<std::string>::const_reverse_iterator it = input.rbegin(); it != input.rend() - 1; ++it)
    oss << *it << ' '; // 3
  oss << input[0]; // 4

  return oss.str(); // 5
}

1. The input vector can't be empty.
2. I cared more about readability than performances here. Otherwise I would have be tempted to use a plain STL string, reserving to it enough memory on creation (summing up all the string sizes plus the number of blanks).
3. Put to the string stream all the elements in the vector followed by a blank, starting from the last one (reverse begin) up the the second (the one before the reverse end). We have asserted that the vector has at least an element, so know that this loop works fine. In case there is only one string in input, this block is simply skipped.
4. The first element in input (possibly the only one), is put to the string stream as last token. No trailing blank, as required.
5. Extract the resulting string from the stream and return it.