在C ++中，调用我的函数以根据程序的需要返回不同的类型时，是否可以做一些时髦的事情？

Question

Consider the useful code by Zaita posted at cplusplus.com , in particular the part which gets numbers safely, modified to be a function in my case:

int get_number()
{
   /**
    * cplusplus.com/forum/articles/6046
    * gets number from input stream
    **/

   string input = "";
   int number = 0;

   while (true)
   {
      getline(cin, input);
      stringstream checks(input);
      if (checks >> number)
         return number;
      cout << "Please enter a valid number\n";
   }
}

Now, my question is this: Can I remove the int on the first line of the function definition for get_number(), and declare it at the top of my code with all the types I might want to return such as doing some declarations like this:

double get_number();
int get_number();
long get_number();
unsigned short get_number();
...
...

And somehow get it to do different returns depending on the variable I want to store the return from the function with? Currently I simply writing multiple definitions of essentially the same function while changing the name to get_someType

I am hoping I can do something like declare with this sort of syntax:

int get_number(int);
double get_double(double);
...
...

And my desire would be to do something like:

int x;
x = get_number(int);

I am sure this will NOT work however! Because it would be impossible to define the function's source code with parameters with no names...

Answer 1

Firstly, the get_number implementation you've found is badly written: what do you think happens when you get an EOF on input? It spins around printing "Please enter a valid number\\n" as fast as it can until you kill the program....

Secondly, you can't overload on return type - which means you can't have functions that only differ in the return type.

Finally, C++ has templates that can do what I think you want...

template <typename T>
bool get(T& x)
{
    std::string input;
    if (!getline(std::cin, input))
        return false;
    stringstream checks(input);
    if (checks >> x)
        return true;
    cout << "Please enter a valid value\n";
}

You can then use if (get(my_int)) ... or if (get(my_double)) etc..

As input can not be guaranteed to succeed, you really should either provide a boolean return type (as illustrated) or throw an exception once it's clear input can not succeed.

Answer 2

You're looking for function template s !

This code is buggy (please don't use cplusplus.com as a reference), since it doesn't check that input from std::cin is received.

A templated example would look something like this (untested):

#include <string>
#include <sstream>
#include <iostream>
#include <stdexcept>

template <typename T>
T get_number()
{
    std::string input = "";
    T number; // Don't initialise to zero
    bool done = false;

    while (!done)
    {
        if (std::getline(std::cin, input)) // check for success
        {
            std::istringstream checks(input);
            if (checks >> number)
            {
                done = true;
            }
            else
            {
                std::cout << "Please enter a valid number\n";
            }
        }
        else
        {
            throw std::runtime_error("No input was received\n");
        }
    }
    return number;
}

int main()
{
    try
    {
        int i = get_number<int>();
        float f = get_number<float>();
        double d = get_number<double>();
        char b = get_number<char>();
    }
    catch (const std::runtime_error& e)
    {
        std::cerr << e.what() << std::endl;
    }
}

To incorporate my comment below into this answer... In a class template or function template, typename can be used as an alternative to class to declare templated types. I prefer typename because we're dealing with POD-types and not class es.

Answer 3

This can be done with templates.

Here you can find out some template tutorials for beginners (start from 58..) http://thenewboston.org/list.php?cat=16

Answer 4

You can use templates, but as far as I know your function must have an argument of the type of that template as well, as in:

template <class T>
T get_number(T arg)
{
   /**
    * cplusplus.com/forum/articles/6046
    * gets number from input stream
    **/

   string input = "";
   T number = 0;

   while (true)
   {
      getline(cin, input);
      stringstream checks(input);
      if (checks >> number)
         return number;
      cout << "Please enter a valid number\n";
   }
}

The type of template ( T ) is resolved in compile time based on the type of argument passed to the function.

Answer 5

template<class T>
T get_number()
{
   /**
    * cplusplus.com/forum/articles/6046
    * gets number from input stream
    **/

   string input = "";
   T genericobj;

   while (true)
   {
      getline(cin, input);
      stringstream checks(input);
      if (checks >> genericobj)
         return genericobj;
      cout << "Please enter a valid input\n" ;
   }
}