Problem with format and initialization

Peter1234 (26)
I have to create a class containing both a const and a non-const float. Initialize these using the constructor initializer list.

I cannot find direct examples, and still have problems with “const float ff2” and relevant function.
Assuming that FFloat1 is correctly defined, I would be grateful if someone could comment why the output for all the print options is 3, except for
printf("float =%f\n", FFloat1(ff1) )
The answer for it is
float = 0.000000

include <iostream>
#include <iomanip>
using namespace std;

class MyString {
float ff1;
//const float ff2; -------------------- Does not work
public:
FFloat1(float ff1);
//FFloat2(const float ff2); -------------------- Does not work
void print();
};

MyString::FFloat1(float ff1){ff1 = 3.141633; return ff1;};
//MyString:FFloat2(const float ff2) {}; -------------------- I do not know how to do this
void MyString::print()
{
cout << "output 1 "<< FFloat1(ff1) << endl;
cout << " output 2 "<< setw(6) << FFloat1(ff1) << endl;
cout << " output 3 "<< setw(-6) << FFloat1(ff1) << endl;
cout << " output 4 "<< setiosflags(ios::left) << setw(20) << FFloat1(ff1) << endl;
cout << " output 5 "<< setw(13) << setprecision(6) << FFloat1(ff1) << endl;
printf("float =%f\n", FFloat1(ff1) );
}



int main()
{

MyString ST;

ST.print();
system("Pause");
}
Last edited on
guestgulkan (2942)
Assume we class like this:
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class MyClass
{
    const float ff2;
};


Then the constant variables must be initialized in an initialization list. This means that you must have at least a programmer provided constructor because the compiler will be unable to provide a default one.

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class MyClass
{
    const float ff2;
    
    public:
    MyClass () //constructor
    :ff2(10.9f) //initialize constant objects in initialization list - see Note 1 below
    {}
};


Note 1.
Static constant integral members can be initialized in the class declaration
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class MyClass
{
   static  const int i = 3; 
};
Peter1234 (26)
Many thanks to guestfulkan for his/her explanation. How can the example be extended to use constructor initializer list? I tried the following without success

class MyClass
{
public:
MyClass();

private:
float ff1;
const float ff2;
};

MyClass::MyClass()
: ff1(_ff1),
ff2(10.9f)
{ }

guestgulkan (2942)
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MyClass::MyClass()
: ff1(_ff1), //what is _ff1
ff2(10.9f)
{ }


PS - use the # button when posting code - so that it shows up like above
Peter1234 (26)
My dificulty in using constructor initializer list with a combination of constant and non-constant variables was resolved with line 20. Is this the only way of formulating for such variables? 


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class MyClass
{
public:
MyClass(float _ff1): ff1(_ff1),ff2(10.9f){ };
void print20a();

private:
float ff1;
const float ff2;
};

void MyClass::print20a()
{
cout << "MyClass ff1   << ff1 << endl;
cout << "MyClass ff2   << ff2 << endl;
}

int main()
{
MyClass ST20a = MyClass(3.23415);   
ST20a.print20a();
}

output:
MyClass ff1 3.23415
MyClass ff2 10.9
guestgulkan (2942)
The problem here is that you have no default constructor (one that takes no parameters) BUT you have a constructor that take one or more parameters MyClass(float _ff1) then you (the programmer) is responsible to provide the default constructor.

You probably found that you could not do this:
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int main()
{
MyClass ST20a; //ERROR - cannot create a variable using default constructor
ST20a.print20a();
}


so you did this instead:
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int main()
{
MyClass ST20a  = MyClass(3.23415);//the compiler would do this  - MyClass ST20a (MyClass(3.23415)) - copy construction
ST20a.print20a();
}
Peter1234 (26)
Many thanks to guestgulkan (896) for the explanation
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