CODEWITHSUNDEEP
c++
Brandon
Brandon

Reputation: 23498

Weird behaviour with substring

I'm writing an algorithm to decode base64. In the below code near the very end, if I change:

Binary.substr((FirstChar - 1) >= 0 ? (I - 1) : 0);

to

Binary.substr((I - 1) >= 0 ? (I - 1) : 0);

It throws std::out_of_range. However, if I leave it alone, it works fine..

The entire code is as follows:

#include <iostream>
#include <bitset>
#include <algorithm>

static const std::string Base64Chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

std::string DecodeBase64(std::string Data)
{
    std::string Binary = std::string();
    std::string Result = std::string();

    for (std::size_t I = Data.size(); I > 0; --I)
    {
        if (Data[I - 1] != '=')
        {
            std::string Characters = Data.substr(0, I);
            for (auto it = Characters.begin(); it != Characters.end(); ++it)
                Binary += std::bitset<6>(Base64Chars.find(*it)).to_string();
            break;
        }
    }

    for (std::size_t I = 0; I < Binary.size(); I += 8)
    {
        int FirstChar = I;
        std::string str = Binary.substr((FirstChar - 1) >= 0 ? (I - 1) : 0);
        Result += static_cast<char>(std::bitset<8>(str).to_ulong());
        if (I == 0) ++I;
    }

    return Result;
}

int main()
{
    std::cout<<DecodeBase64("aGVsbG8gdGhlcmUgbm9vYg==");
}

It is weird because I assigned I to FirstChar right before I call substr so it should be the same exact value.. Any ideas why this is happening?

Upvotes: 2

Views: 154

Answers (1)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726479

This is because I is of type std::size_t, which is unsigned. When I is zero, I - 1 is interpreted as a very large positive number.

Converting I to an int which happens in the assignment fixes the problem, because FirstChar is now signed, so FirstChar -1 could become negative.

Converting I-1 >= 0 to an equivalent I >= 1 should fix this problem:

Binary.substr(I >= 1 ? (I - 1) : 0);

Upvotes: 6

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