String tolower()

The problem here is converting an entire C++ string to lower (or upper) case.

The raw part of the job is done by the toupper/tolower ctype functions in the standard library. The point is that those functions work on a single character, we should find a way of iterating on the entire string.

We could just loop from the beginning to the end of the string, applying tolower() to each element in the sequence. Something like this:

for(std::string::iterator it = line.begin(); it != line.end(); ++it)
  *it = std::tolower(*it);

This implementation is alright, still we could avoid to explicitly do some work that nothing add as a value to our code, as declaring the iterator and a for-loop block.

We could use instead the STL transform() algorithm, that hides the for-block and its loop variable, asking as input the begin-end iterators for the input sequence, the begin iterator to the output one, and the operation it has to apply to transform it.

Here we are doing an in-place transformation, so input and output are the same.

Nice and plain. There is a tiny nuisance, though. There could be a name-clash on tolower/toupper, since besides the ctype version, there is also a localized version, defined in the locale include. So we can't simply say to the compiler we want to use the standard tolower function, we need to specify which tolower is needed. In our case, the one that expects an int as both input parameter and return value:

std::transform(line.begin(), line.end(), line.begin(), static_cast<int(*)(int)>(std::tolower));