从文件中读取并交换c++中的特定字符

在这个函数中，我需要将输入的文件中的所有字符(例如a)替换为输入的另一个字符(例如i)。我已经试过两次了，但作为新人，我的大脑已经太迟了，没有什么建议了吗?

void swapping_letter()
{
ifstream inFile("decrypted.txt");   
char a;
char b;
string line;
if (inFile.is_open())
{
    while (!inFile.eof())
    {
        getline(inFile,line);
    }
    cout<<"What is the letter you want to replace?"<<endl;
    cin>>a;             
    cout<<"What is the letter you want to replace it with?"<<endl;
    cin>>b;
    replace(line.begin(),line.end(),a,b);

            inFile<<line

    inFile.close();
}
else
{
    cout<<"Please run the decrypt."<<endl;
}
}

或:

void swapping_letter()
{
ifstream inFile("decrypted.txt");   
char a;
char b;
if (inFile.is_open())
{
    const char EOL = 'n';                                          
    const char SPACE = ' ';
    cout<<"What is the letter you want to replace?"<<endl;
    cin>>a;             
    cout<<"What is the letter you want to replace it with?"<<endl;
    cin>>b;
    vector<char> fileChars;                                     
    while (inFile.good())                                            
    {
        char c;
        inFile.get(c);
        if (c != EOL && c != SPACE)                             
        {
            fileChars.push_back(c);
        }

        replace(fileChars.begin(),fileChars.end(),a,b);
        for(int i = 0; i < fileChars.size(); i++)
        {
            inFile<<fileChars[i];
        }
    }
}
else
{
    cout<<"Please run the decrypt."<<endl;
}
}