将char值添加到字符串后，char值变为负值

Question

我的问题是从十六进制包计算十进制字符后出了什么问题，字符串中的字符在将它们添加到字符串后都有负值，你能不能看一下这段代码，我会非常高兴每一个解决方案你提供

#include <vector>
#include <iostream>
#include <string>

// this function in to calculating a sha1 hash in decimal
std::string sha1_bits(const std::string &calc_before) // <-- calc_before = sha1 hash
{
    // the final string, which will be returned at the end of the function
    std::string r = "";

    // the hexCharset in right order
    const std::string hexaDezimalCharset = "0123456789abcdef";
    // some constants
    const int hexMultiplier = 16, hexpair = 2, shaInBitChar = 20;


    int tempNum = 0;
    std::vector<std::string> storage;

    // resize the storage vector to 20
    // there will be 20 chars, out of 40 char hex sha1 hash
    storage.resize(shaInBitChar);

    for(int i = 0; i < storage.size(); i++)
    {
        storage.at(i).resize(hexpair);
    }

    // put them in 20 strings, each of them has to chars with is forming 1 char when decoding in dezimal
    for(int i = 0; i < calc_before.size(); i++)
    {
        storage.at((i - (i % hexpair)) / hexpair).at(i % hexpair) = calc_before.at(i);
    }

    // Print the hex packages, just for debug
    for(int i = 0; i < storage.size(); i++)
    {
        for(int p = 0; p < storage.at(i).size(); p++)
        {
            std::cout << storage.at(i).at(p);
        }
        std::cout << std::endl;
    }
    std::cout << std::endl;

    // for all doubled hex packages ... 
    for(int i = 0; i < storage.size(); i++)
    {
        // the tempNum is the final decimal value decoded from hex package
        tempNum = 0;
        for(int q = 0; q < storage.at(i).size(); q++)
        {
            for(int p = 0; p < hexaDezimalCharset.size(); p++)
            {
                // if the char is found in the hexCharset ...
                if(hexaDezimalCharset.at(p) == storage.at(i).at(q))
                {
                    // and it's the first char in the package
                    if(q == 0)
                    {
                        // the index will be multiplied with 16 and added to tempNum
                        tempNum += p * hexMultiplier;
                    }
                    else if(q == 1) // if its the second one
                    {
                        // the index will just added to tempNum
                        tempNum += p;
                    }
                    break;
                }
            }
        }

        // the tempNum will be added to r, after it was converted to char format
        r.push_back(char(tempNum));
    }

    return r;
}

int main()
{
    std::string theReturnedString = sha1_bits("b37a4f2cc0624f1690f64606cf385945b2bec4ea");
    // But some chars extracted from the returned string have negative values
    // Everything i could figure out is that the Numbers are fine, which are forming and are stored in tempNum
    // the numbers only get negative if the hex package from which it formed began with an character instead of a number
    // at this point i notice that i'm quite unconcetrated and my english is getting a mess
    // shortly:
    // hex package (storage) --> decimal (tempNum) --> put at the end of the string ( r.at( x ) )
    // "b3" --> 179 --> -77
    // "7a" --> 122 --> 122
    // "4f" --> 79 --> 79
    // maybe to escape my bad description you should take this code, put it into your compiler or http://cpp.sh, check out what's happening and 

    for(int i = 0; i < theReturnedString.size(); i++)
    {
        std::cout << (int) theReturnedString.at(i) << std::endl;
    }
    return 0;
}