我应该使用什么函数数据类型来返回 vector.push_back()？

Question

string yesno(string holder ,vector<string> symptom)
{
    if(holder == "Y" || holder == "y"){
        return symptom.push_back("Y");
    }
    else if(holder == "N" || holder == "n"){
        return symptom.push_back("N");
    }
    else{
        cout <<"Wrong input" << endl;
        return symptom.push_back("-");
    }
}

So this is my function for when I'm asking a user are they experiencing a certain symptom and then ask for Y/y or N/n for an answer.

The if blocks in the function detects the upper and lower case of Y or N and then I'm returning symptom.push_back() as "Y" or "N" but I'm getting this error:

error: could not convert 'symptom.std::vectorstd::__cxx11::basic_string<char ::push_back(std::__cxx11::basic_string(((const char*)"Y"), std::allocator()))' from 'void' to 'std::__cxx11::string' {aka 'std::__cxx11::basic_string'}|

Answer 1

The std::vector.push_back() function has a void return type, so there is no way you can convert what it returns to any type (because it doesn't return anything).

However, there is the std::vector.back() function, which returns a reference to the last element; so, you could make your conditional push_back() calls in the relevant blocks, then have a single return statement that 'retrieves' the element that was pushed:

string yesno(string holder, vector<string> symptom)
{
    if (holder == "Y" || holder == "y") {
        symptom.push_back("Y");
    }
    else if (holder == "N" || holder == "n") {
        symptom.push_back("N");
    }
    else {
        cout << "Wrong input" << endl;
        symptom.push_back("-");
    }
    return symptom.back();
}

Also, as noted in the comments, you are passing your symptom vector by value , which means that a local copy will be used and any changes made to that will be lost when the function returns (the copy will be deleted). You may want to consider passing symptom by reference:

string yesno(string holder, vector<string>& symptom) // Pass by reference!
{
    //...