C++ Inheritance with strings

Question

So I found this problem on Inheritance I am try to figure out. this is what's required:

Create a class called strMetric that will give information about a string. You should provide a default constructor and an overloaded constructor that takes a string as an argument.

Your string metric class should have the following functioality

A method called howLong that returns the length of the string

A method called vowelCnt that returns the number of vowels in a string

A method called charSum that returns the sum of all characters within the string

A method called upperCase that returns the number of upper case characters

A method called lowerCase that returns the number of lower case characters

You are to use the strMetric class as the derived class and use the string class as the base class.

NOTE:

Do not not create your own string class and derive from it. You are to use the string class that is part of the std namespace and defined in

I have been working this out, and this is what I have, (right now I'm only working on 1 of the methods until I figure out how to do this properly)

//// strmetric.h ////

#ifndef STRMETRIC
#define STRMETRIC
#include <string>
using namespace std;

class strMetric : public string
{
private:

        public:
        strMetric();
        strMetric(string &s);
        int howLong(string s);
};
#endif

//// strmetric.cpp ////

#include "strmetric.h"

strMetric::strMetric()
    :string()
{
}
strMetric::strMetric(string &s)
    :string(s)
{
}

int strMetric::howLong(string s)
{ 
    return s.size();
}

/////main.cpp////

#include <iostream>
#include "strmetric.h"

strMetric testRun("Hello There");

int main()
{

cout << "Here is the sentence being tested " << endl;
cout << endl;
cout << testRun << endl;
cout << endl;
cout << "String length " << endl;
cout << endl;
cout << testRun.length(testRun) << endl;
cout << endl;

}

So am I doing this correctly, or am I way off base? I'm having a hard time getting my head around this. If I'm doing it incorrectly could somebody show me how to do it correctly, I don't need the whole thing, just the one part I started so I can get a good idea on what I should be doing, thanks!

Basic rule: never inherit from std classes. Wrap them instead. — Tom van der Woerdt
– Tom van der Woerdt, Commented Apr 18, 2013 at 23:53
Wow, they're actually telling you to inherit from std::string? — Joseph Mansfield
– Joseph Mansfield, Commented Apr 18, 2013 at 23:53
I think you could have string as your member variable. It's not a good idea to inherit from it. — gongzhitaao
– gongzhitaao, Commented Apr 18, 2013 at 23:53
@JBentley: Not true (unfortunately): "You are to use the strMetric class as the derived class and use the string class as the base class." - and then: "Do not not create your own string class and derive from it. You are to use the string class that is part of the std namespace and defined in". What a dumb assignment. — Ed Swangren
– Ed Swangren, Commented Apr 18, 2013 at 23:54
@MichellePeoples: Look up "Composition". You would have a private std::string member and use it to do what you need, but you would not inherit from it. Your class is not a string, therefore should not inherit from it. However, this is all irrelevant in terms of your assignment as you have to do so. You can just politely tell your teacher that all of SO thinks s/he should re-evaluate the assignment as it is teaching you bad, bad things. — Ed Swangren
– Ed Swangren, Commented Apr 18, 2013 at 23:55

Sergey Kalinichenko · Accepted Answer · 2013-04-19 00:57:52Z

4

Leaving aside a generally good advice to not inherit from std containers, preferring composition instead, you are still not doing it right: the method howLong (and the rest of the methods that they expect you to define) are supposed to operate on the string itself, not on a string passed in:

int howLong(); // no args

int strMetric::howLong() {
    // Using "this" below is optional
    return this->size(); // remember, this *is* a string
}

The rest of the methods will do the same thing - they will not take a string argument, and use this instead.

How in the world can you do the other methods using this-> instead of loading in a string argument?

The remaining methods are not different - simply take the string inside your own object. I am nearly certain that learning how to do this is the point of the exercise. For example, to add vowel counter, you do something like this:

int vowelCnt(); // again, no args

int strMetric:: vowelCnt() {
    int res = 0;
    for (int i = 0 ; i != this->size() ; i++) {
        char ch = (*this)[i]; // Yes, this works
        if (ch == 'a' || ch == 'u' || ch == ...) {
            res++;
        }
    }
    return res;
}

edited Apr 19, 2013 at 0:57

answered Apr 18, 2013 at 23:57

Sergey Kalinichenko

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8 Comments

Michelle Peoples Over a year ago

So in my header I initialize it without any overloads, and then overload it with a string in my cpp file, and then use this to cal on the string from the string class?

Ed Swangren Over a year ago

@MichellePeoples: No, no overloads. You just don't take any arguments. I'm not sure why das did the example this way and left the argument in. Probably just a typo.

Michelle Peoples Over a year ago

A better question would be, how in the world can you do the other methods using this-> instead of loading in a string argument? I can do them all loading in either chars *s or string s, but I don't think I've ever seen it done with a this-> pointer

Sergey Kalinichenko Over a year ago

@MichellePeoples Take a look at the update to see how it can be done. I think you were missing the (*this)[i] part.

Michelle Peoples Over a year ago

Ok, so I have this for counting the string in ASCII int charSum(string x, int i) { if(x.size() == i) return (0 + x[i]); else return charSum(x, i + 1) + x[i]; } and I am trying to do it the way your saying, and going with this:

int strMetric::sumChars() { 	int x = 0;   if(this->size() == x) 	  return (0 + (*this)[x]);   else     return this->x + 1 + (*this)[x]; }

But it does not return the current count for the characters being scanned, any idea why?

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