Thread: Unresolved external on vector inside class constructor

Having problems vector again. I'm studying static data members and doing a toy class to further my knowledge. This class has a single data member and several static members. I placed a few comments to better illustrate the class. The idea... to have a class that records the sum and average of its objects.

Code:

class counter {
public:
    counter(int);
    
    inline int value() const;  //number getter

    //static members getters
    inline static unsigned long count();
    inline static unsigned long sum();
    inline static double avg();
    static double ravg(unsigned long x); //pass vector index, return avg.
private:
    int number;

    static unsigned long count_; //object count
    static unsigned long sum_;  //sum of counter::number
    static double avg_;              //average of counter::number
    static std::vector<double> ravg_;  //holds running average
};

/* inline definitions are here */

The constructor is being defined on the class cpp file. Also shown, the static members initialization.

Code:

unsigned long counter::count_ = 0;
unsigned long counter::sum_ = 0;
double counter::avg_ = 0;
std::vector<double> ravg_(1, 0);

counter::counter(int x = 0): number(x) {
    count_ += 1;
    sum_ += x;
    avg_ = static_cast<double>(sum_) / static_cast<double>(count_);
    ravg_.push_back(avg_); //unresolved external error
}

Why am I getting an unresolved external in the constructor pointing to the ravg_.push_back() line?

Originally Posted by Mario F.

Having problems vector again. I'm studying static data members and doing a toy class to further my knowledge. This class has a single data member and several static members. I placed a few comments to better illustrate the class. The idea... to have a class that records the sum and average of its objects.

Code:

class counter {
public:
    counter(int);
    
    inline int value() const;  //number getter

    //static members getters
    inline static unsigned long count();
    inline static unsigned long sum();
    inline static double avg();
    static double ravg(unsigned long x); //pass vector index, return avg.
private:
    int number;

    static unsigned long count_; //object count
    static unsigned long sum_;  //sum of counter::number
    static double avg_;              //average of counter::number
    static std::vector<double> ravg_;  //holds running average
};

/* inline definitions are here */

The constructor is being defined on the class cpp file. Also shown, the static members initialization.

Code:

unsigned long counter::count_ = 0;
unsigned long counter::sum_ = 0;
double counter::avg_ = 0;
std::vector<double> counter::ravg_(1, 0);

counter::counter(int x = 0): number(x) {
    count_ += 1;
    sum_ += x;
    avg_ = static_cast<double>(sum_) / static_cast<double>(count_);
    ravg_.push_back(avg_); //unresolved external error
}

Why am I getting an unresolved external in the constructor pointing to the ravg_.push_back() line?

See code in red above!

Gah, hk_mp5kpdw beat me to it

*headdesks*

The funny thing about all this is seeing me preaching about namespaces on another thread and forgetting about them in my own code. Tsk!

Thanks both

FWIW, Typically if I had a container of classes containing values that I needed to calculate a sum and average for I'd make a seperate class to hold those statistics and call the for_each STL function on the container of objects instead of making some static variables.

A brief example... something along the lines of:

Code:

class data
{
    int value;
public:
    data( int val = 0 ) : value(val) {}
    int GetValue() const
    {
        return value;
    }
};

class average
{
    int sum;
    int count;
public:
    average() : sum(0), count(0) {}
    void operator()(const data& item)
    {
        ++count;  // Increase number of objects processed
        sum += item.GetValue();
    }
    double GetAverage() const
    {
        return static_cast<double>(sum) / count;
    }
};

...

vector<data> data_vect;

// Fill data_vect with some data objects
data_vect.push_back( data(6) );
data_vect.push_back( data(14) );

// Calculate average of all the objects in data_vect
average avg = for_each(data_vect.begin(), data_vect.end(), average());
cout << "Average of all objects is: " << avg.GetAverage() << endl;

Output should be:

Code:

Average of all objects is: 10

Typically if I had a container of classes containing values that I needed to calculate a sum and average for I'd make a seperate class to hold those statistics and call the for_each STL function on the container of objects instead of making some static variables.

I agree, though wouldnt you prefer std::accumulate to std::for_each for the sum, then use the sum to calculate the average?

Thanks hk_mp5kpdw.

I understood the logic behind your implementation, so I'm sure i'll do just that if I ever need this type of functionality on my code.

I'm getting sick of running into stuff I don't have. My copy of Dec-C++ does not implement this algorithm. Guess I need to download the STL and install it or something.

It's in <numeric>

Originally Posted by citizen

Why. SGI said it was somewhere else.

They lied.

Why. SGI said it was somewhere else.

The SGI STL reference notes that std::accumulate is:
"Defined in the standard header numeric, and in the nonstandard backward-compatibility header algo.h."

Incidentally, if you take the std::accumulate route, it might be something like:

Code:

class SumDataValue
{
public:
    int operator()(int init, const data& item)
    {
	    return init + item.GetValue();
    }
};

// ...

// Calculate average of all the objects in data_vect
int sum = std::accumulate(data_vect.begin(), data_vect.end(), 0, SumDataValue());
double avg = static_cast<double>(sum) / data_vect.size();
cout << "Average of all objects is: " << avg << endl;

Ouch! Just go hold on some info on your implementation and it surely is painful .