Question

I am trying to write a simple function that will get me a number from user input within a range.

When instanciating this function i explicitly tell it i want it instanciated with int but still i get the error:

thermo.cpp:105:31: error: no matching function for call to ‘getInput(int&)’

Why is is trying to find a function that takes int& as argument?

template<class T, T min = std::numeric_limits<T>::min, T max = std::numeric_limits<T>::max>
T getInput(T default_value = T()){
  std::string input;
  T myNumber = T(); //default inits
  while(true){
    getline(cin, input);

    if(input.length() == 0){
      return default_value;
    }

    // This code converts from string to number safely.
    stringstream myStream(input);
    if (myStream >> myNumber){
      if(myNumber > max || myNumber < min){
        stringstream ss;
        ss << "Input out of bounds. Received " << myNumber << " expected between " << min << " and " << max << ".";
        throw invalid_argument(ss.str());
      }
      return myNumber;
    }

    cout << "Invalid number, please try again" << endl;
  }
}

void get(const std::string& prompt, int& param){
  cout << prompt << " [" << param << "]:";
  param = getInput<int,0>(param); // i specifically tell it i want 'int', why am i getting 'int&'?
}

Update

If i try CharlesB suggestion:

void get(const std::string& prompt, int& param){
  cout << prompt << " [" << param << "]:";
  param = getInput<int,0>(int(param));
}

i get

thermo.cpp:105:36: error: no matching function for call to ‘getInput(int)’

Forgot:

g++ 4.5.3 under cygwin

Command line:

$ g++ thermo.cpp -o thermo.exe -Wall -pedantic -std=c++0x

Update 2

if i call it like this

void get(const std::string& prompt, int& param){
  cout << prompt << " [" << param << "]:";
  param = getInput<int,0,15>(int(param)); // fully parameterized
}

it works… but i’d rather not specify an upper bound (not even numeric_limits) on each call.

Answer 1

Don’t use templates for min and max:

template<class T>
T getInput(T default_value = T(), T min = std::numeric_limits<T>::min(), T max = std::numeric_limits<T>::max());

There is no reason to use templates for those arguments(besides the fact that it does not work).

Edit: You can’t use those arguments as template values since std::numeric_limits<T>::min() is a function, its value is known on runtime, and template value arguments have to be bound to a value at compile time. This is valid:

template<class T, T min = 0, T max = 5>
T getInput(T default_value);

Since 0 and 5 are known during compilation.