Converting a for loop to use std::transform instead - c++

Question

I'm working on a function and it currently works as is.
void ceasarShift( CeasarText& text, int amount ) {
// Bound amount to the number of characters in the alphabet
amount %= 26;
for ( std::size_t i = 0; i < text._originalText.length(); i++ ) {
char c = text._originalText[i] + amount;
text._shiftedText += c;
}
}
The class that this function takes is a simple class that has 2 std::string member variables. When this class is passed by reference where this is a friend function to that class, it takes the original string that is already set and uses that to populate the shifted string which is empty before this function sees it. When the for loop is done, the function is finished and the class object is returned back by reference with a newly updated shifted string and the original is unmodified.
I was thinking of maybe using std::transform and a lambda to do the same task. This is what I have tried.
void ceasarShift( CeasarText& text, int amount ) {
// Bound amount to the number of characters in the alphabet
amount %= 26;
/*for ( std::size_t i = 0; i < text._originalText.length(); i++ ) {
char c = text._originalText[i] + amount;
text._shiftedText += c;
}*/
std::transform( text._originalText.begin(), text._originalText.end(),
text._shiftedText.begin(),
[amount]( unsigned char c ) -> unsigned char { return c + amount; }
);
}
And I end up getting a Debug Assertion Failed! Window message Expression: cannot seek string iterator past end
I'm wondering if my lambda is wrong or if I need to use something else other than std::transform.
Edit
I have also tried this and this works like the first loop:
{
amount %= 26;
for ( auto& c : text._originalText )
text._shiftedText += (c + amount);
}
I just can not seem to get the lambda to work properly as the predicate to std::transform.

Answer 1

The problem is not your lambda. The difference between your two snippets, is that your for loop appends elements to your string, using operator+=, but your transform assumes that the elements already exist. If you want to append to the string with transform, then you will need to use an insert iterator. Specifically a back insert iterator. So instead of passing text._shiftedText.begin(), pass std::back_inserter(text._shiftedText).
std::transform( text._originalText.begin(), text._originalText.end(),
std::back_inserter(text._shiftedText),
[amount]( unsigned char c ) -> unsigned char { return c + amount; }
);

Answer 2

According to this link, The behavior of std::transform is equivalent to:
template <class InputIterator, class OutputIterator, class UnaryOperator>
OutputIterator transform (InputIterator first1, InputIterator last1,
OutputIterator result, UnaryOperator op)
{
while (first1 != last1) {
*result = op(*first1); // or: *result=binary_op(*first1,*first2++);
++result; ++first1;
}
return result;
}
which means the container of OutputIterator should reserve enough space, otherwise, it will out of range when ++result