//splits a string into a vector of multiple tokens
std::vector<string> split_str(std::string& str, const char* delimiter){
    std::vector<string> ret;
    size_t currPos = 0;
    //Add the first element to the vector
    if (str.find(delimiter) != string::npos)
        ret.push_back(str.substr(currPos, str.find(delimiter)));

    while (currPos != str.size() - 1){

        if (str.find(delimiter, currPos) != string::npos){
            //Current at one past the delimiter
            currPos = str.find(delimiter, currPos) + 1;
            //Substring everything from one past the delimiter until the next delimiter
            ret.push_back(str.substr(currPos, str.find(delimiter, currPos)));
        }
        //If last whitespace is not right at the end
        else if (currPos < str.size()){
            //Add the last element to the vector and end the loop
            ret.push_back(str.substr(currPos, str.size()));
            currPos = str.size() - 1;
        }

    }
    return ret;
}

The program is supposed to take a string and a delimiter as an input and return a vector of strings (tokens) as an output. However, when I try it with a simple input such as:

ab bc cd de (delimiter is " ")

The output will be 5 elements: "ab", "bc cd", "cd de", "de", "de"

The problem is that second parameter to std::string::substr() is count not position. Your code should be modified from:

if (str.find(delimiter) != string::npos)
    ret.push_back(str.substr(currPos, str.find(delimiter)));

to this:

auto fpos = str.find(delimiter);
if (fpos != string::npos)
    ret.push_back(str.substr(currPos, fpos - currPos));
    //                                ^^^^^^^^^^^^^^

and so on.