Calling a default constructor

Question

I need the user to input two bool values in main(), which then will be inputed as arguments in a class function found in a separate file. However, I also would like the user to invoke the default constructor in the class if no bool values are inputed. How can I do that?

Set appropriate default values on the variables that will be the paramters for the constructor. In case the user does not input parameters, forward those values to the constructor, — MFH
– MFH, Commented Jul 8, 2012 at 23:03
Have user enter bool1 and bool2 in main(). Call on a class function which takes the two values as arguments. If no input, set bool1 and bool2 to some default value. EDIT: not homework. I'm just curious. — Nico Bellic
– Nico Bellic, Commented Jul 8, 2012 at 23:05

MFH · Accepted Answer · 2012-07-08 23:26:49Z

1

Considering the following situation:

struct Test {
  bool val1, val2;

  Test(bool val1, bool val2) : val1(val1), val2(val2) { }
};

The cleanest solution would look somewhat like this:

int main() {
  bool val1 = true, val2 = true;//init with defaults
  if(userWantsToChangeDefaults) std::cin >> val1 >> val2;//obviously this is more or less pseudocode
  Test test(val1, val2);
}

answered Jul 8, 2012 at 23:26

MFH

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mathematician1975 · Accepted Answer · 2012-07-08 23:28:00Z

1

Implement a constructor that takes default arguments. For example

  TestClass(bool bVal1 = false, bool bVal2 = false);

where you change as required to implement your desired default behaviour, Then if you create an instances like this

  TestClass test_instance;   // constructor uses default args
  TestClass test_instance2(true,false);  // constructor uses specified args

then the constructor will treat the arguments as the default values set in the constructor declaration (if none are provided).

Alternatively have 2 constructors

  TestClass();
  TestClass(bool bVal1, bool bVal2);

and let the user of the class use whichever is appropriate.

edited Jul 8, 2012 at 23:28

answered Jul 8, 2012 at 23:08

mathematician1975

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3 Comments

MFH Over a year ago

Whilst that works, it's 100% easier and cleaner to use default values in the calling code and supply these to the constructor in case the user doesn't input data...

mathematician1975 Over a year ago

@MFH You could well be right. The question is not 100% clear. Why not write up your proposed method as an answer so the OP has more options.

Qaz Over a year ago

You should really lose the brackets on TestClass test_instance();, lest you declare a function.

Community · Accepted Answer · 2021-01-18 12:35:56Z

Just to substantiate my hint 'use a parser', here is a very very simple (read: lacking all input validation) approach, based on Boost Spirit.

You could write this with plain IO Streams, and you'd get basically what @MFH posted. Depending on the context, the simpler approach might be more adequate.

#include <boost/fusion/adapted.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/support_istream_iterator.hpp>

struct MyClass
{
    bool _a, _b;
    MyClass(bool a, bool b) :_a(a), _b(b) { }
    MyClass(){}
};

BOOST_FUSION_ADAPT_STRUCT(MyClass,(bool,_a)(bool,_b));

namespace qi    = boost::spirit::qi;

int main()
{
    std::cin.unsetf(std::ios::skipws);
    boost::spirit::istream_iterator f(std::cin), l;

    MyClass data;
    using namespace qi;

    auto optbool = bool_ | attr(false);

    if (phrase_parse(f,l, 
            (optbool >> optbool),
            qi::blank, data))
    {
        std::cout << "data._a: " << std::boolalpha << data._a << "\n"
                  << "data._b: " << std::boolalpha << data._b << "\n";
    } else
        std::cerr << "Illegal input\n";
}

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