Question

Two questions for an assignment, first is the first set of errors for my template in a the CharComparator.h file, I don’t see why it is complaining. The second question is why that I added include “CharComparator.h” to my main.cpp file, I get a bunch more of compiler errors. Here are my files:

CharComparator.h

#ifndef CHARCOMPARATOR_H
#define CHARCOMPARATOR_H

template <> 
int my_comp<const char*>(const char *a, const char *b) {
    return std::strcmp(a, b) < 0;
}

#endif

main.cpp

// Reduce Template Assignment

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

#include "CharComparator.h"

// definition of global varible
const int MAX = 10;

// declaration of template functions
template <class T>
int reduce(T array[], int size);

template <class T>
void show(const T array[], int size);

// Main program for testing
int main() {
    // test using long instantiation
    long nonUniqueArray[MAX] = {12, 12 ,5, 6, 11, 5, 6, 77, 11, 12};

    // show non-unique array
    cout << "old array with non-unique elements: " << endl;
    show(nonUniqueArray, MAX);

    int newsize = reduce(nonUniqueArray, MAX);

    // now non-unique array becomes unique
    cout << "new array has only unique elements: " << endl;
    show(nonUniqueArray, newsize);
    cout << "size reduced to " << newsize << endl;
    cout << endl;

    // test using string instantiation
    const char* strArray[MAX] = {"aa", "bb", "bc", "ca", "bc", "aa", "cc", "cd", "ca", "bb"};
                                //aa bb bc ca cc cd
    // show non-unique array
    cout << "string array with non-unique elements: " << endl;
    show(strArray, MAX);

    newsize = reduce(strArray, MAX);

    // now non-unique array becomes unique
    cout << "string array has only unique elements: " << endl;
    show(strArray, newsize);
    cout << "size reduced to " << newsize << endl;

    return (0);
}



// reduce the non-unique array to unique array, return new size

template <class T> 
int reduce(T array[], int size) {
// CODE UP A REDUCE TEMPLATE HERE
    T *begin = array;
    T *end = array + size;
    sort(begin, end);
    T *end_new = unique(begin, end);
    return end_new - array;
}

// show the array element
template <class T>
void show(const T array[], int size) {
    for (int i = 0; i < size; i++) {
        cout << array[i] << ' ';
    }

    cout << endl;
}

And of course, compiler errors:

08\projects\c4\c4_1b_jwong\c4_1b_jwong\charcomparator.h(5) : error C2143: syntax error : missing ';' before '<'
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\charcomparator.h(5) : error C2988: unrecognizable template declaration/definition
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\charcomparator.h(5) : error C2059: syntax error : '<'
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\main.cpp(22) : error C2065: 'MAX' : undeclared identifier
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\main.cpp(22) : error C2078: too many initializers
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\main.cpp(26) : error C2065: 'MAX' : undeclared identifier
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\main.cpp(28) : error C2065: 'MAX' : undeclared identifier
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\main.cpp(37) : error C2065: 'MAX' : undeclared identifier
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\main.cpp(37) : error C2078: too many initializers
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\main.cpp(41) : error C2065: 'MAX' : undeclared identifier
1>c:\users\jon\documents\visual studio 2008\projects\c4\c4_1b_jwong\c4_1b_jwong\main.cpp(43) : error C2065: 'MAX' : undeclared identifier
1>Build log was saved at "file://c:\Users\jon\Documents\Visual Studio 2008\Projects\C4\C4_1b_JWong\C4_1b_JWong\Debug\BuildLog.htm"
1>C4_1b_JWong - 11 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Any thoughts? Thanks, and sorry for such noob quesitons.

Answer 1

If you want to specialize a template function, you need to have at least declared the original template first, i.e.:

template<class T> int my_comp<T>(T a, T b);

// ... now you can specialize my_comp()

Specializations are for providing implementations for “special cases” of a generic template, see e.g. C++ FAQ lite 35.7. As James points out there are some intricacies to template specialization to be aware of that Sutter described in this article.