当我编译时，我不断得到构建失败？ 我正在使用 Xcode

Question

I don't know why this won't work. I don't know if it have to do with passing though the function or the way I created my variables. But I have to use the toDigit function for my homework.

#include <iostream>
#include <fstream>
using namespace std;

int toDigit(char &);

int main(){
    int number;
    char isbn;

    ifstream inputFile;
    inputFile.open("numbers.txt");

    while (inputFile >> isbn) {
        number = toDigit(isbn);
        cout << number << endl;
    }
    inputFile.close();
    return 0;
}

int toDigit(char ch){
    return ch - '0';
}

Answer 1

Initially, you declare toDigit to be

 int toDigit(char &);

which would be read as this: a function returning an int that takes one argument that is a reference to a char.

Then, later you define this function like this:

int toDigit(char ch){
    return ch - '0';
}

which could be read like this: a function returning an int that takes one argument that is a char.

Do you see the difference? Hint: a reference .

In c++, definitions need to be the same as declarations, so you need to make them the same.

EITHER:

Make the declaration

 int toDigit(char );
 //               ^^ no &

OR

make the definition

int toDigit(char & ch){
//               ^^ & added
    return ch - '0';
}

In this case, the first option makes more sense because you don't need read access the the character and chars are efficient to copy, so a copy would be fine.