I'm trying to read a file into a vector and keep getting this error

Question

I keep getting this error:

 error: no match for ‘operator>>’ (operand types are ‘std::ifstream’
 {aka ‘std::basic_ifstream<char>’} and ‘std::vector<symbol>’)
   26 |             myFile >> temp;
      |             ~~~~~~ ^~ ~~~~

int readAlphabetFromFile(string filename, vector<symbol> alphabet) {
    ifstream myFile;
    myFile.open(filename);
    if (myFile.is_open()) {
        while (!myFile.eof()) {
            vector<symbol> temp;
            myFile >> temp;
            alphabet.push_back(temp);
        }
        return 0;
    }
    else {
        return -1;
    }
}

you can't read dirctly into a vector. I think you want to read into `symbol temp;` (Although that depends on the definition of symbol. — AShelly, Oct 24 '22 at 01:01
the while-not-eof is wrong https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-i-e-while-stream-eof-cons — Neil Butterworth, Oct 24 '22 at 01:06
Unless you offered an overload for your structure, the >> operator can't read into a structure. Same for a vector of your structure. Your compiler must be trying you that none of this is defined! — Marcus Müller, Oct 24 '22 at 01:06
You must think that C++ is black magic and could intelligently do whatever you ask it to do, but no, it doesn't know how to do `>>` for your custom-defined `symbol`, in fact, it doesn't know to do anything unless you specifically teach it how to do so. [C++ ifstream and ofstream overloading operator reading from file](https://stackoverflow.com/q/38232496/13253010) — thedemons, Oct 24 '22 at 01:20
You need to change `vector temp;` to `symbol temp;` and then implement `operator>>` for `symbol`. Also, `alphabet` needs to be passed in by reference. — Remy Lebeau, Oct 24 '22 at 01:31

score 1 · Answer 1 · answered Oct 24 '22 at 01:23

myFile >> temp;

In C++ an overloaded >> operator is defined for just a few elementary types and classes; namely it's defined for all numerical and floating point types, std::string, and a few other simple things.

This temp is a std::vector of some unknown type named symbol.

There is no defined overload of the >> operator for a vector of anything. Additionally if you defined a class named symbol there's not going to be a >> overload of that, either.

while (!myFile.eof()) {

Another problem with the shown code is that this is always a bug.

There are very few things in C++ that work by default. If your goal is to read a vector of these symbols, whatever they are, you will need to write all the logic and all the code to do that, step by step, all by yourself. C++ will not do it for you. You will also need to use appropriate logic for checking for an end of file condition, as explained in the preceding link.

Remy Lebeau · Answer 2 · 2022-10-24T01:43:56.477

The error message is telling you that the compiler can't find an operator>> for reading a std::vector from a std::ifstream, which is true.

However, alphabet is a vector of symbols, so you need to push symbol objects into it, but you are trying to push another vector instead. You need to change vector<symbol> temp; to just symbol temp; and then implement operator>> for symbol.

Also, alphabet needs to be passed in to your function by reference or pointer, but you are passing it in by value, so you are acting on a copy of the caller's vector, not the original.

Also, while (!myFile.eof()) is a logic error that needs to be fixed: Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong?

Try this instead:

istream& operator>>(istream &in, symbol &sym) {
    // read from 'in' into 'sym' as needed...
    return in;
}

int readAlphabetFromFile(string filename, vector<symbol> &alphabet) {
    ifstream myFile(filename);
    if (myFile.is_open()) {
        symbol temp;
        while (myfile >> temp) {
            alphabet.push_back(temp);
        }
        return 0;
    }
    else {
        return -1;
    }
}

I'm trying to read a file into a vector and keep getting this error

2 Answers2