在函数中返回ifstream [英] Returning ifstream in a function

问题描述

这可能对您来说是一个非常讨厌的问题:如何(如果可能的话)从函数返回ifstream?

Here's probably a very noobish question for you: How (if at all possible) can I return an ifstream from a function?

基本上，我需要从用户那里获取数据库的文件名，如果不存在具有该文件名的数据库，则需要为用户创建该文件.我知道该怎么做，但只能在创建文件后要求用户重新启动程序.我想尽可能避免给用户带来不便，但是下面的函数无法在gcc中编译:

Basically, I need to obtain the filename of a database from the user, and if the database with that filename does not exist, then I need to create that file for the user. I know how to do that, but only by asking the user to restart the program after creating the file. I wanted to avoid that inconvenience for the user if possible, but the function below does not compile in gcc:

ifstream getFile() {
    string fileName;
    cout << "Please enter in the name of the file you'd like to open: ";
    cin >> fileName;
    ifstream first(fileName.c_str());
    if(first.fail()) {
        cout << "File " << fileName << " not found.\n";
        first.close();
        ofstream second(fileName.c_str());
        cout << "File created.\n";
        second.close();
        ifstream third(fileName.c_str());
        return third; //compiler error here
    }
    else
        return first;
}

抱歉，忘记告诉您编译器错误的位置和原因:

sorry, forgot to tell you where and what the compiler error was:

main.cpp:45: note: synthesized method ‘std::basic_ifstream<char, std::char_traits<char> >::basic_ifstream(const std::basic_ifstream<char, std::char_traits<char> >&)’ first required here 

我更改了函数，而不是按照Remus的建议返回一个指针，并将main()中的行更改为"ifstream database = * getFile()"；现在我再次收到此错误，但这一次在main()中的行中:

I changed the function to return a pointer instead as Remus suggested, and changed the line in main() to "ifstream database = *getFile()"; now I get this error again, but this time in the line in main():

main.cpp:27: note: synthesized method ‘std::basic_ifstream<char, std::char_traits<char> >::basic_ifstream(const std::basic_ifstream<char, std::char_traits<char> >&)’ first required here

推荐答案

bool checkFileExistence(const string& filename)
{
    ifstream f(filename.c_str());
    return f.is_open();
}

string getFileName()
{
    string filename;
    cout << "Please enter in the name of the file you'd like to open: ";
    cin >> filename;
    return filename;
}

void getFile(string filename, /*out*/ ifstream& file)
{
    const bool file_exists = checkFileExistence(filename);
    if (!file_exists) {
        cout << "File " << filename << " not found." << endl;
        filename = getFileName();  // poor style to reset input parameter though
        ofstream dummy(filename.c_str();
        if (!dummy.is_open()) {
            cerr << "Could not create file." << endl;
            return;
        }
        cout << "File created." << endl;
    }
    file.open(filename.c_str());
}

int main()
{
    // ...
    ifstream file;
    getFile("filename.ext", file);
    if (file.is_open()) {
        // do any stuff with file
    }
    // ...
}

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