Converting a number from one base to another in C++

ConvertBase.hpp

// NOTE: For the sake of practice, STL/stdlib helpers are not used, just std::string for memory management.

#include <string>
using std::string;

// Convert arbitrary base to base 10
// - Input must be valid number in the given base
// - Base must be between 2 and 36
// - If input or base are invalid, returns 0
long ConvertTo10(const string& input, int base)
{
    if(base < 2 || base > 36)
        return 0;
    
    bool isNegative = (input[0] == '-');    

    int startIndex = input.length()-1;
    int endIndex   = isNegative ? 1 : 0;
    
    long value = 0;
    int digitValue = 1;
    
    for(int i = startIndex; i >= endIndex; --i)
    {
        char c = input[i];
        
        // Uppercase it - NOTE: could be done with std::toupper
        if(c >= 'a' && c <= 'z')
            c -= ('a' - 'A');
        
        // Convert char to int value - NOTE: could be done with std::atoi
        // 0-9
        if(c >= '0' && c <= '9')
            c -= '0';
        // A-Z
        else
            c = c - 'A' + 10;
        
        if(c >= base)
            return 0;
        
        // Get the base 10 value of this digit    
        value += c * digitValue;
        
        // Each digit has value base^digit position - NOTE: this avoids pow
        digitValue *= base;
    }
    
    if(isNegative)
        value *= -1;
    
    return value;
}

// Convert base 10 to arbitrary base
// - Base must be between 2 and 36
// - If base is invalid, returns "0"
// - NOTE: this whole function could be done with itoa
string ConvertFrom10(long value, int base)
{
    if(base < 2 || base > 36)
        return "0";
    
    bool isNegative = (value < 0);
    if(isNegative)
        value *= -1;
    
    // NOTE: it's probably possible to reserve string based on value
    string output;
    
    do
    {
        char digit = value % base;
     
        // Convert to appropriate base character
        // 0-9
        if(digit < 10)
            digit += '0';
        // A-Z
        else
            digit = digit + 'A' - 10;
        
        // Append digit to string (in reverse order)
        output += digit;
        
        value /= base;
        
    } while (value > 0);
    
    if(isNegative)
        output += '-';
    
    // Reverse the string - NOTE: could be done with std::reverse
    int len = output.size() - 1;
    for(int i = 0; i < len; ++i)
    {
        // Swap characters - NOTE: Could be done with std::swap
        char temp = output[i];
        output[i] = output[len-i];
        output[len-i] = temp;
    }
    
    return output;
}

// Convert from one base to another
string ConvertBase(const string& input, int baseFrom, int baseTo)
{
    // NOTE: There is probably a more efficient way to convert between two bases.
    //       This however is easy to understand and debug.
    return ConvertFrom10(ConvertTo10(input, baseFrom), baseTo);
}

main.cpp

#include <iostream>
using std::cout;
using std::endl;

int main()
{
    // Bases 10 and 2 (positive)
    cout << "===== 1 =====" << endl;
    cout << ConvertBase("101",  2,  10) << endl; // 100(2)   = 5(10)
    cout << ConvertBase("5",   10,   2) << endl; // 5(10)    = 101(2)

    // Bases 10 and 16 (negative)
    cout << "===== 2 =====" << endl;
    cout << ConvertBase("-1A",  16, 10) << endl; // -1A(16)  = -26(10)
    cout << ConvertBase("-26",  10, 16) << endl; // -26(10)  = -1A(16)

    // Bases 5 and 7
    cout << "===== 3.1 =====" << endl;
    cout << ConvertBase("100",   5, 10) << endl; // 100(5)   = 25(10)
    cout << ConvertBase("25",   10,  7) << endl; // 25(10)   = 34(7)
    // So therefore...
    cout << "===== 3.2 =====" << endl;
    cout << ConvertBase("100",   5,  7) << endl; // 100(5)   = 34(7)
    cout << ConvertBase("34",    7,  5) << endl; // 34(7)    = 100(5)
 
    // Larger Bases (26, 13)
    cout << "===== 4 =====" << endl;
    cout << ConvertBase("1234", 26, 13) << endl; // 1234(26) = 8864(13)
    cout << ConvertBase("8864", 13, 26) << endl; // 8864(13) = 1234(26)
 
    return 0;   
}

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You should probably build the output string in reverse order. e.g. instead of: output += digit; try: output = digit + output;

P.S. You will also have to do the same for the sign. output += '-'; Should be: output = '-' + output;

I actually don't recommend doing this.

output = digit + output;
and even
output = output + digit;
Are both significantly slower operations than
output += digit;
when it comes to the std string data type.

Meanwhile, std:reverse is actually extremely performant.
So it's ultimately faster to build strings in reverse order using += and then reverse them using std::reverse.