用C++实现RC4算法，要求： 1）实现字符串加密解密功能； 2）或者实现文件加密解密功能； 3）或者实现文件分割为3个子文件再加密生成3个密文文件，3个密文文件解密恢复为加密前原文件。 说明：密钥至少8个字符，文件与可执行程序放在一个目录，如果是字符文件取ASCII值。

以下是用C++实现RC4算法的示例代码，实现了字符串加密解密功能：

#include <iostream>
#include <cstring>
#include <cstdlib>

using namespace std;

const int MAXN = 256;

void KSA(unsigned char* key, int keylen, unsigned char* S) {
    for (int i = 0; i < MAXN; i++) {
        S[i] = i;
    }
    int j = 0;
    for (int i = 0; i < MAXN; i++) {
        j = (j + S[i] + key[i % keylen]) % MAXN;
        swap(S[i], S[j]);
    }
}

void PRGA(unsigned char* S, unsigned char* data, int datalen, unsigned char* out) {
    int i = 0, j = 0;
    for (int k = 0; k < datalen; k++) {
        i = (i + 1) % MAXN;
        j = (j + S[i]) % MAXN;
        swap(S[i], S[j]);
        out[k] = data[k] ^ S[(S[i] + S[j]) % MAXN];
    }
}

void RC4(unsigned char* key, int keylen, unsigned char* data, int datalen, unsigned char* out) {
    unsigned char S[MAXN];
    KSA(key, keylen, S);
    PRGA(S, data, datalen, out);
}

int main() {
    // 读入明文和密钥
    string plaintext, key;
    cout << "请输入明文: ";
    getline(cin, plaintext);
    cout << "请输入密钥(至少8个字符): ";
    getline(cin, key);
    if (key.length() < 8) {
        cout << "密钥长度不足8个字符!" << endl;
        return 0;
    }

    // 明文和密钥转换为二进制形式
    int textlen = plaintext.length();
    int keylen = key.length();
    unsigned char* data = new unsigned char[textlen];
    unsigned char* keydata = new unsigned char[keylen];
    for (int i = 0; i < textlen; i++) {
        data[i] = plaintext[i];
    }
    for (int i = 0; i < keylen; i++) {
        keydata[i] = key[i];
    }

    // 加密
    unsigned char* ciphertext = new unsigned char[textlen];
    RC4(keydata, keylen, data, textlen, ciphertext);
    cout << "密文: ";
    for (int i = 0; i < textlen; i++) {
        printf("%02X", ciphertext[i]);
    }
    cout << endl;

    // 解密
    unsigned char* plaintext2 = new unsigned char[textlen];
    RC4(keydata, keylen, ciphertext, textlen, plaintext2);
    cout << "解密后明文: " << plaintext2 << endl;

    delete[] data;
    delete[] keydata;
    delete[] ciphertext;
    delete[] plaintext2;
    return 0;
}

在该示例代码中，我们首先利用KSA算法初始化S数组，然后利用PRGA算法生成密钥流，最后将密钥流和明文进行异或得到密文。解密过程与加密过程相同。

如果要实现文件加密解密功能，可以将上述代码中的字符串替换为文件读写操作，将加密后的内容写入到一个新文件中，解密时再将新文件的内容进行解密并写入到原文件中。

如果要实现文件分割为3个子文件再加密生成3个密文文件，3个密文文件解密恢复为加密前原文件，需要进行一些额外的操作。具体步骤如下：

1. 将原文件分为3个子文件

FILE* fp1 = fopen(filename, "rb");
if (fp1 == NULL) {
    cout << "打开文件失败!" << endl;
    return 0;
}
FILE* fp2 = fopen("file1.dat", "wb");
FILE* fp3 = fopen("file2.dat", "wb");
FILE* fp4 = fopen("file3.dat", "wb");
if (fp2 == NULL || fp3 == NULL || fp4 == NULL) {
    cout << "创建文件失败!" << endl;
    return 0;
}
int count = 0;
while (!feof(fp1)) {
    char ch = fgetc(fp1);
    if (count % 3 == 0) {
        fputc(ch, fp2);
    } else if (count % 3 == 1) {
        fputc(ch, fp3);
    } else {
        fputc(ch, fp4);
    }
    count++;
}
fclose(fp1);
fclose(fp2);
fclose(fp3);
fclose(fp4);

2. 对3个子文件进行加密操作

unsigned char* keydata = new unsigned char[keylen];
for (int i = 0; i < keylen; i++) {
    keydata[i] = key[i];
}
FILE* fp1 = fopen("file1.dat", "rb");
FILE* fp2 = fopen("file2.dat", "rb");
FILE* fp3 = fopen("file3.dat", "rb");
FILE* fp4 = fopen("ciphertext1.dat", "wb");
FILE* fp5 = fopen("ciphertext2.dat", "wb");
FILE* fp6 = fopen("ciphertext3.dat", "wb");
if (fp1 == NULL || fp2 == NULL || fp3 == NULL
    || fp4 == NULL || fp5 == NULL || fp6 == NULL) {
    cout << "打开文件失败!" << endl;
    return 0;
}
unsigned char* data = new unsigned char[blocksize];
unsigned char* ciphertext = new unsigned char[blocksize];
while (!feof(fp1) || !feof(fp2) || !feof(fp3)) {
    int len1 = fread(data, sizeof(unsigned char), blocksize, fp1);
    int len2 = fread(data + len1, sizeof(unsigned char), blocksize - len1, fp2);
    int len3 = fread(data + len1 + len2, sizeof(unsigned char), blocksize - len1 - len2, fp3);
    int total = len1 + len2 + len3;
    RC4(keydata, keylen, data, total, ciphertext);
    int len4 = fwrite(ciphertext, sizeof(unsigned char), total, fp4);
    int len5 = fwrite(ciphertext + len1, sizeof(unsigned char), total - len1, fp5);
    int len6 = fwrite(ciphertext + len1 + len2, sizeof(unsigned char), total - len1 - len2, fp6);
    if (len4 != total || len5 != total - len1 || len6 != total - len1 - len2) {
        cout << "写入文件失败!" << endl;
        return 0;
    }
}
fclose(fp1);
fclose(fp2);
fclose(fp3);
fclose(fp4);
fclose(fp5);
fclose(fp6);
delete[] data;
delete[] ciphertext;

3. 对3个密文文件进行解密操作

unsigned char* keydata = new unsigned char[keylen];
for (int i = 0; i < keylen; i++) {
    keydata[i] = key[i];
}
FILE* fp1 = fopen("ciphertext1.dat", "rb");
FILE* fp2 = fopen("ciphertext2.dat", "rb");
FILE* fp3 = fopen("ciphertext3.dat", "rb");
FILE* fp4 = fopen("file.dat", "wb");
if (fp1 == NULL || fp2 == NULL || fp3 == NULL || fp4 == NULL) {
    cout << "打开文件失败!" << endl;
    return 0;
}
unsigned char* data = new unsigned char[blocksize];
unsigned char* plaintext = new unsigned char[blocksize];
while (!feof(fp1) || !feof(fp2) || !feof(fp3)) {
    int len1 = fread(data, sizeof(unsigned char), blocksize, fp1);
    int len2 = fread(data + len1, sizeof(unsigned char), blocksize - len1, fp2);
    int len3 = fread(data + len1 + len2, sizeof(unsigned char), blocksize - len1 - len2, fp3);
    int total = len1 + len2 + len3;
    RC4(keydata, keylen, data, total, plaintext);
    int len4 = fwrite(plaintext, sizeof(unsigned char), total, fp4);
    if (len4 != total) {
        cout << "写入文件失败!" << endl;
        return 0;
    }
}
fclose(fp1);
fclose(fp2);
fclose(fp3);
fclose(fp4);
delete[] data;
delete[] plaintext;

需要注意的是，文件分割和合并的操作只是示例代码，实际应用时需要更加完善和严密。