C++ specialization function

newbie0101 · 05-26-2012, 01:31 PM

hello, i am working on programming tasks form book but i am stuck at one task where i don't clearly understand what to do and have no idea how to do it. i've done only part of the problem.
here's question(sorry for my engish, it's not my native lang.):

Code:

This part i've done:
write template of function maxn() that takes two arguments,
first is array of type T
second is an integer representing number of items in the array.
the function returns its biggest value.
Use the template in the program that uses the function template with array of six ints and four doubles.

At this point i am stuck:
the program should contain specialization that takes two args.
first: array of pointers to char
second: number of pointers.
it should return address of the longest string.
test the specialization with array of five strings pointers

here's my code:

Code:

#include <iostream>

using namespace std;

template <typename T>
T maxn(T arr[], int n);

void maxn<char>(char * arr[], int n);

int main()
{
    int integers[6] = {1,3,6,4,0,2};
    double dble[4] = {193.69, 207.02, 0.09, 20.1};
    
    cout << "Max value from int is: ";
    cout << maxn(integers, 6) << endl;
    cout << "Max value from double is: ";
    cout << maxn(dble, 4);
}

template <typename T>
T maxn(T arr[], int n)
{
    T max = arr[0];
    for (int i = 1; i < n; i++)
        if (arr[i] > max)
            max = arr[i];
    return max;
}

can you show me a way how to do it ?

pan64 · 05-26-2012, 01:43 PM

the template has comparison operator, so arr[i] > max will call the comparator of the specific type. Using strings the comparator is working based on the alphabetical order. You need the longest one, so you need to modify how this comparator should work.

newbie0101 · 05-27-2012, 07:38 AM

i know how to figure out which string is the longest one but i don't know what the declaration should look like. can you show me one ?
you can see i've tried one but i am not sure if this is correct:

Code:

void maxn<char>(char * arr[], int n);

millgates · 05-27-2012, 02:50 PM

Hi, how about something like

Code:

template <> void maxn < char > (char * arr[], int n);

That's a template specialization. Alternatively, you can try

Code:

void maxn(char * arr[], int n);

That's a non-template function. Non template functions are preferred to templates of the same name.

pan64 · 05-28-2012, 04:59 AM

I do not really understand:

Code:

...
int main()
{
    int integers[6] = {1,3,6,4,0,2};
    double dble[4] = {193.69, 207.02, 0.09, 20.1};
    string strings[5] = { "asdf", "qwer", "retyhrw", "3q4trc", "34rcq" };
    
    cout << "Max value from int is: ";
    cout << maxn(integers, 6) << endl;
    cout << "Max value from double is: ";
    cout << maxn(dble, 4) << endl;
    cout << "Max value from strings is: ";
    cout << maxn(strings, 5) << endl;
}

this code works well with strings using the same maxn, just it will return the last one not the longest one.
The problem is the comparison operator, you need to override it.
So create a new class: mystring:

Code:

class mystring: public string {
    public: mystring(const char a[]): string(a) {};
    friend bool operator> (mystring &s1, mystring &s2);
};

bool operator> (mystring &s1, mystring &s2) {return s1.length()>s2.length(); };

and use this one instead of the default string class.

(the line: void maxn<char>(char * arr[], int n); not needed)

johnsfine · 05-28-2012, 06:46 AM

Quote:

Originally Posted by newbie0101

the program should contain specialization

That assignment has poor wording, because you can't specialize a template function. You overload it instead. You only specialize template classes.

Quote:

void maxn<char>(char * arr[], int n);

That is incorrect. You want to just overload the function.

Quote:

Originally Posted by millgates

how about something like

Code:

template <> void maxn < char > (char * arr[], int n);

That's a template specialization.

Yes, that is a template specialization and I've even seen compilers accept that. But if I understand correctly, that is not valid C++. I think the standard says you can't fully specialize a templated function.

Quote:

Alternatively, you can try

Code:

void maxn(char * arr[], int n);

That's a non-template function. Non template functions are preferred to templates of the same name.

That is overloading the function, which so far as I understand is the only correct approach to this situation.

pan64 · 05-28-2012, 07:58 AM

this is what you are looking for?

Code:

#include <iostream>
#include <string.h>

using namespace std;

template <typename T> T maxn(T arr[], int n);
template <> char * maxn(char * arr[], int n);

int main()
{
    int integers[6] = {1,3,6,4,0,2};
    double dble[4] = {193.69, 207.02, 0.09, 20.1};
    string strings[5] = { "asdfrtyurty", "qwer", "retyhrw", "3q4trc", "34rcq" };
    char * chars[5] = { "asdfrtyurty", "qwer", "retyhrw", "3q4trc", "34rcq" };
    
    cout << "Max value from int is: ";
    cout << maxn(integers, 6) << endl;
    cout << "Max value from double is: ";
    cout << maxn(dble, 4) << endl;
    cout << "Max value from strings is: ";
    cout << maxn(strings, 5) << endl;
    cout << "Max value from chars is: ";
    cout << maxn(chars, 5) << endl;
}

template <typename T>
T maxn(T arr[], int n)
{
    T max = arr[0];
    for (int i = 1; i < n; i++)
        if (arr[i] > max)
            max = arr[i];
    return max;
}


template <>
char * maxn<char *>(char *arr[], int n)
{
    unsigned max = strlen(arr[0]);
    char * r = arr[0];
    for (int i = 1; i < n; i++) {
        if (strlen(arr[i]) > max) {
            max = strlen(arr[i]);
            r = arr[i];
        }
    }
    return r;
}

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millgates · 05-28-2012, 10:28 AM

Quote:

Originally Posted by johnsfine

Yes, that is a template specialization and I've even seen compilers accept that. But if I understand correctly, that is not valid C++. I think the standard says you can't fully specialize a templated function.

It compiles with g++ -Wall -Wextra -pedantic -ansi without errors. I do not have a copy of the standard (Is one available somewhere? Probably not.), so I can't tell for sure what the standard says about it. I never really used this, actually. But I have seen this used (I believe even in some tutorials). That does not mean it's correct, of course

Actually, isn't it the other way around: function templates can be fully specialized but not partially specialized?

dwhitney67 · 05-29-2012, 06:55 AM

It seems the OP is MIA (perhaps because of the holiday weekend in the US?)

Here's my take:

Code:

#include <iostream>
#include <cstring>

template <typename T>
T& lexicocomp(T& lhs, T& rhs)
{
    return lhs > rhs ? lhs : rhs;
}

template <typename T>
T& charstrcomp(T& lhs, T& rhs)
{
    return strcmp(lhs, rhs) > 0 ? lhs : rhs;
}

template <typename T>
T& lengthcomp(T& lhs, T& rhs)
{
    return strlen(lhs) > strlen(rhs) ? lhs : rhs;
}

template <typename T>
T maxn(T arr[], int n, T& (*compare)(T& lhs, T& rhs) = lexicocomp)
{
    T max = arr[0];

    for (int i = 1; i < n; i++)
    {
        max = compare(arr[i], max);
    }

    return max;
}

int main()
{
    using namespace std;

    int integers[6]      = { 1, 3, 6, 4, 0, 2 };
    double dble[4]       = { 193.69, 207.02, 0.09, 20.1 };
    string strings[5]    = { "asdfrtyurty", "qwer", "retyhrw", "3q4trc", "34rcq" };
    const char* chars[5] = { "asdfrtyurty", "qwer", "retyhrw", "3q4trc", "34rcq" };

    cout << "Max value from int is: ";
    cout << maxn(integers, 6) << endl;

    cout << "Max value from double is: ";
    cout << maxn(dble, 4) << endl;

    cout << "Max value from strings is: ";
    cout << maxn(strings, 5) << endl;

    cout << "Max value from chars is: ";
    cout << maxn(chars, 5, charstrcomp) << endl;

    cout << "Max length from chars is: ";
    cout << maxn(chars, 5, lengthcomp) << endl;
}