C++：将向量传递给 function 然后在 main 中调用 function。 遗漏了什么

Question

#include <iostream>
#include <string>
#include <ctime>
#include <cstdlib>
#include <vector>

using namespace std;

void getAnswer(std::vector<std::string> &answers, int nAnswers)
{
    int index = rand() % nAnswers;

}

int main()
{
    std::vector<string> answers;
    answers.push_back("Most Certainly"); 
    answers.push_back("Absolutely"); 
    answers.push_back("Yes"); 
    answers.push_back("You Can Bet On It"); 
    answers.push_back("Odds look good"); 
    answers.push_back("Let's talk about that some other time"); 
    answers.push_back("Odds don't look so good"); 
    answers.push_back("I think you know the answer to that question"); 
    answers.push_back("I don't think I'm qualified to answer that question"); 
    answers.push_back("Absolutely Not"); 
    answers.push_back("I Don't Think So");
    answers.push_back("Um...no");

    std::vector<string> qAnswers(answers);

    answers.size();

    string questionAsked;
    bool pgExit = false;


    srand((unsigned int)time(NULL));

    cout << "\nWelcome to the Magic 8Ball.\n";
    cout << "\nAsk a question and I will predict the answer!\n" << endl;

    //loop and ask the user to enter a question or enter "x" to stop
    while (!pgExit) {

        cout << "What is your question? (Type question or Enter 'x' to exit) " << endl;

        //use getline to get the question
        getline(cin, questionAsked);

        //call getAnswer with your array and number of possible answers to get an answer
        getAnswer(answers, answers.size());

        //output the answer
        if (questionAsked.compare("x") == 0)
        {
            cout << "Maybe next time. Have a good day.";
            pgExit = true;

        }
        if (questionAsked.compare("") != 0 && questionAsked.compare("x") != 0)
        {
            getAnswer;
            std::cout << getAnswer(answers, answers.size()) << std::endl;
        }
    } 

}

The issue I am having is when I compile, it is saying 'no operator matches "<<" these operands. Standard operands are: std::ostream << void'

I am not sure I understand. I am passing the vector string and the vector size to void getAnswers to get the randomize for the answers. What am I missing?

Any help is greatly appreciated.

Answer 1

void getAnswer(std::vector<std::string> &answers, int nAnswers)

The void return "type" states explicitly that this function returns nothing so you cannot then go and attempt to use that return value in an expression:

std::cout << getAnswer(answers, answers.size()) << std::endl;

Assuming that you will eventually return a random answer from your list of answers (based on code to date), the first thing you should do is dummy up a reurn value:

std::string getAnswer(std::vector<std::string> &answers, int nAnswers)
{
    // Just return last one for now (make sure you
    // have at least one in the collection).

    return answers[nAnswers - 1];
}

Then you can later adapt it to provide a random one. I could have just provided the complete function but, since this is probably educational work, you'll learn a lot more by doing this yourself (this answer is just to get you over your specific problem).

I have included some sample code at the bottom for you to look over (and for future readers who may not be doing the classwork) but I urge you to try yourself first.

---

As an aside, you should also be aware that you have some rather superfluous lines in your code, specifically:

answers.size();
getAnswer(answers, answers.size());
getAnswer;

None of these do anything useful, they simply evaluate the expression and throw away the result.

I'm also not why you attempt to create a second vector from the original, especially since you don't use it anywhere:

std::vector<string> qAnswers(answers);

---

As mentioned earlier, my sample code follows. Please do not use it verbatim if you value your marks (or integrity) - any educator worth their salt will be using plagiarism detection tools:

#include <iostream>
#include <string>
#include <ctime>
#include <cstdlib>
#include <vector>

// Anon namespace to hide this in translation unit.

namespace {
    // No need to pass in size, vector has this.

    const std::string &getAnswer(const std::vector<std::string>answers) {
        return answers[std::rand() % answers.size()];
    }
};

int main() {
    srand(static_cast<unsigned int>(time(nullptr)));

    // Can be const if initialised rather than each entry pushed.

    const std::vector<std::string> answers = {
        "Most Certainly", "Absolutely", "Yes", "You Can Bet On It",
        "Odds look good", "Let's talk about that some other time",
        "Odds don't look so good",
        "I think you know the answer to that question",
        "I don't think I'm qualified to answer that question",
        "Absolutely Not", "I Don't Think So", "Um...no",
    };

    std::cout << "\nWelcome to the Magic 8Ball.\n";
    std::cout << "\nAsk a question and I will predict the answer!\n";

    // Infinite loop, use break to exit.

    while (true) {
        // Ask and get response.

        std::cout << "\nWhat is your question (x to exit)? " << std::endl;
        std::string questionAsked;
        std::getline(std::cin, questionAsked);

        // Exit loop if told so.

        if (questionAsked == "x") {
            std::cout << "Maybe next time. Have a good day.\n\n";
            break;
        }

        // Answer any non-blank question.

        if (! questionAsked.empty()) {
            std::cout << getAnswer(answers) << '\n';
        }
    }
}