C++base64解码

以下是使用C++进行Base64解码的示例代码：

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

std::string base64_decode(const std::string& encoded_string) {
    const std::string base64_chars =
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz"
        "0123456789+/";

    std::vector<unsigned char> decoded_bytes;
    int in_len = encoded_string.size();
    int i = 0;
    int j = 0;
    int in_ = 0;
    unsigned char char_array_4[4], char_array_3[3];

    while (in_len-- && (encoded_string[in_] != '=') && (isalnum(encoded_string[in_]) || (encoded_string[in_] == '+') || (encoded_string[in_] == '/'))) {
        char_array_4[i++] = encoded_string[in_];
        in_++;
        if (i == 4) {
            for (i = 0; i < 4; i++) {
                char_array_4[i] = base64_chars.find(char_array_4[i]);
            }

            char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
            char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
            char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

            for (i = 0; i < 3; i++) {
                decoded_bytes.push_back(char_array_3[i]);
            }
            i = 0;
        }
    }

    if (i) {
        for (j = i; j < 4; j++) {
            char_array_4[j] = 0;
        }

        for (j = 0; j < 4; j++) {
            char_array_4[j] = base64_chars.find(char_array_4[j]);
        }

        char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
        char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
        char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

        for (j = 0; j < i - 1; j++) {
            decoded_bytes.push_back(char_array_3[j]);
        }
    }

    std::string decoded_string(decoded_bytes.begin(), decoded_bytes.end());
    return decoded_string;
}

int main() {
    std::string encoded_string = "5bCP5piO";
    std::string decoded_string = base64_decode(encoded_string);
    std::cout << "Decoded string: " << decoded_string << std::endl;

    return 0;
}