push_back() in std::vector<std::string> overwriting current string - c++

Question

I'm trying to implement an undo function when inserting a new line for a very simple text editor as a project for class. In the program, the column is where the cursor currently is, and the row is the current line the vector is on.
I was able to successfully create my "insertNewLine()" function which uses a std::vector<std::string> that is able to display the text to the screen. This is how I implemented it:
void Editor::insertNewLine()
{
// get a substring
prevLine = lines[row - 1];
size_t tempSize = prevLine.size();
int lengthOffset = getSubstringOffset(tempSize, (column - 1));
std::string cutTemp = prevLine.substr((column - 1), lengthOffset);
lines[row - 1].erase(column - 1);
// after incrementing, row and amount of lines, initialize the new row
row++;
numberOfLines++;
column = 1;
lines.push_back(cutTemp); // insert substring into new line
}
Here is an example of what the current output looks like this (where | is the cursor):
hello world| (user enters hello world, column = 11, row = 1)
hello|world (user moves cursor to column 5, still on row 1)
(user presses button that calls insertNewLine())
hello
|world (splits where the cursor is to a new line, cursor begins at column 1)
Now, I am able to undo any other command, but when trying to undo a new line, I need to have the cursor return to the previous column, and push the word back where it originally was. I tried implementing that by doing this:
void Editor::undoNewLine()
{
std::string source = lines[row - 1]; // save current line
lines[row-1].clear(); // clear current line
row--; // revert up one row
numberOfLines--; // revert amount of lines
lines.push_back(source); // append
}
With this function, I expected the output to look like this (from the example above):
(user presses a button that calls undoNewLine())
hello|world
But, the problem is, this is the output I get from the current code:
(user presses a button that calls undoNewLine())
|world
Essentially, using push_back(source) overwrites whatever was originally there and brings the cursor to the front. I tried to increment column to the original position it was in before the undo stage, however, this didn't work either. I just ended up with this output:
(user presses a button that calls undoNewLine())
world|
So how should I try and implement this undo function? Any tips or ideas on what I'm doing wrong?

Answer 1

In your solution, you're erasing the previous line's content (hello) with the call to clear(). Instead, just append the current line. The string class makes this easy:
lines[row-1] += lines[row];
After that you can remove the current line with vector::erase.
Note
Be aware that this is might be inefficient, since all the lines below need to be repositioned.
If this does become an issue you can switch to std::list but then you lose random access to your lines.