c++ - Error C2664: Cannot convert argument from initializer list to std:initializer list

Question

I have a function read() that reads a file and store information into a map. However, whenever the function calls map.insert(), it gives me an error.

Employee and Volunteer are two customs classes with only a few variables. For example, If I call

ifstream fin;
std::map<std::string, Employee*> employees;
fin.open("Employee.txt");
read<Employee, EMPLOYEE_SIZE>(fin, employees);

It gives the following error:

std::_Tree<std::_Tmap_traits<_Kty,_Ty,_Pr,_Alloc,false>>::insert(std::initializer_list<std::pair<const std::string,Employee *>>)': cannot convert argument 1 from 'initializer list' to 'std::initializer_list<std::pair<const std::string,Employee *>>'`. 

Here is the function(Both Employee and Volunteer has the same base class:)

template <typename T, int T_LINE_SIZE>
inline void read(std::ifstream& in, std::map<std::string, T*>& input) {
    std::string name = "";
    std::string ID="";
    std::string line;
    double salary;
    while (getline(in,line)) {
        if (typeid(T) == typeid(Volunteer)) {
            name = line.substr(0, 19);
            ID = line.substr(20, T_LINE_SIZE);
            input.insert({name,new Volunteer(ID,name)}); //error happens here
        }else if (typeid(T) == typeid(Employee)) {
            name = line.substr(0, 19);
            ID = line.substr(20, 29);
            salary = std::stod(line.substr(30, T_LINE_SIZE));
            input.insert({ name,new Employee(ID, name, salary) }); //error happens here
        }
    }
}

What am I doing wrong here?

question from:https://stackoverflow.com/questions/65879925/error-c2664-cannot-convert-argument-from-initializer-list-to-stdinitializer-li

Answer 1

Try following code:

template <typename T, int T_LINE_SIZE>
inline void read(std::ifstream& in, std::map<std::string, T*>& input) {
    std::string name = "";
    std::string ID="";
    std::string line;
    double salary;
    while (getline(in,line)) {
        if constexpr (std::is_same<T, Volunteer>::value) {
            name = line.substr(0, 19);
            ID = line.substr(20, T_LINE_SIZE);
            input.insert({name,new Volunteer(ID,name)}); 
        }else if constexpr (std::is_same<T, Employee>::value) {
            name = line.substr(0, 19);
            ID = line.substr(20, 29);
            salary = std::stod(line.substr(30, T_LINE_SIZE));
            input.insert({ name,new Employee(ID, name, salary) }); 
        }
    }
}

The reason is that you are using templates. Based on the type of T when you call read() function, type of T is incorrect in scope of one if condition for calling insert() method. This happens because all possible outcomes of code need to be checked at compile time for different types. But when you use if constexpr, that part of code is ignored at compile time, so incorrect part of code will not be seen during compiling code and your code compiles correctly.