Function not returning desired string

Question

#include <iostream>
#include <string>

using namespace std;
string wordB(string input);

int main() {
    //ask for word
    cout << "Enter a word\n";

    //get word
    string input = "";
    cin >> input;

    //return with b in between all letters
    cout << wordB(input);
    cout << endl << input;
}

string wordB(string str) {
    string rString = "";

    for (unsigned i = 0; i < str.length(); ++i) {
        rString += "B" + str.at(i);
    }
    cout << endl << rString;

    return rString;
}

Trying to display the word a users enter where between every character there is the letter "B". When I run this with the word "join" I get back "trtr".

Answer 1

"B" + str.at(i); doesn't do what you seem to think it does; it's not string conctatenation. It says: take a char* pointer pointing to the beginning of the string literal "B", advance it by the number of characters equal to the ASCII code of character str.at(i), and treat the resulting pointer as pointing to a nul-terminated string. Unless str.at(i) happens to be '\x0' or '\x1' (unlikely), your program exhibits undefined behavior.

There are many different ways to do what you want. Here's one:

rString.push_back('B');
rString.push_back(str[i]);

Answer 2

A particularly nice fix, available from C++14 onwards, is to write

rString += "B"s + str.at(i);

noting the s, which is a user-defined literal. That then forces the overloaded + operator on std::string to be used, rather than the built-in +, which is actually performing dubious (and potentially undefined) pointer arithmetic on the const char[2] literal "B" decayed to a const char*.