Question

Why isn’t inplace_merge merging the strings in the code below?

string src = "abc";
string new_str = "def";
src += new_str;
inplace_merge(src.begin(), src.begin()+3, src.end());
cout << src; // abcdef

Edit: I expected “adbecf”

Answer 1

It does work properly, but you have chosen bad test data.

std::inplace_merge does a merge on two sorted consecutive ranges in a sequence, like in a merge sort.

That is, if you pass the string adebcf (supposing the parameters are set as in your question), it will be merged into abcdef.
If you pass a string abcdef, the result is the same as the input, because this string is already in order.