Help with template specialization

Can someone else help spot what is wrong with the following code:

Code:

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//prototypes
template <class Any>
Any maxn(Any ar[], int n);

template <> Any maxn(char* ar[], int n);

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Here are the errors I get with Dev C++:
11: expected constructor, destructor, or type conversion before "maxn"
11: expected `;' before "maxn"
43: expected constructor, destructor, or type conversion before "maxn"
43: expected `;' before "maxn"

I am trying to create a specialization for the pointer-to-char type.

You specify the Any template parameter as the return type & first parameter of the function, but when you specialized it you only specialized the first parameter as a char*[] but not the return type.

Quote:

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I am trying to create a specialization for the pointer-to-char type.

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Then you need to change your code, because right now it's specialized on an array of strings, not a single string.

Quote:

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Originally Posted by cpjust

You specify the Any template parameter as the return type & first parameter of the function, but when you specialized it you only specialized the first parameter as a char*[] but not the return type.


Then you need to change your code, because right now it's specialized on an array of strings, not a single string.

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Here is the entire code:

Code:

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//prototypes
template <class Any>
Any maxn(Any ar[], int n);

template <> Any maxn(char ar[], int n);

int main() {
    using namespace std;
    int box1[6] = {5, 7, 3, 1, 8, 14};
    double box2[4] = {34.54, 12.656, 78.44, 66.08};
    char * box3[5] = {"Hello World!", "It's a great day.", "The weather is sunny.",
                        "How are you?", "Have a good day."};
   
    cout << "Use maxn() function for integers. Largest element in the array ";
    cout << "of integers is: " << maxn(box1, 6) << endl;
   
    cout << "Use maxn() function for double values. Largest element in the array ";
    cout << "of double values is: " << maxn(box2, 4) << endl;
   
    cout << "Use maxn() function for array of char *. Return address of the longest ";
    cout << "string: " << maxn(box3, 5) << endl;
   
    system("pause");
    return 0;
}

template <class Any>
void maxn(Any ar[], int n) {
    using namespace std;
    Any temp = ar[0];
    for(int i = 1; i < n; i++) {
        if(temp < ar[i]) {
            temp = ar[i]; } }
    return temp;
}

template <> Any maxn(char ar[], int n) {
    using namespace std;
    char * temp = ar[0];
    for(int i = 1; i < n; i++) {
        if(strlen(temp) < strlen(ar[i])) {
            temp = ar[i]; } }
    return *temp;
}

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The specialization for the array of pointer-to-char needs to return the address of the longest string.

If you want an array of strings, then char *ar[] is fine. But you need to return something specific, presumably a char *.

Ok, I changed the return type to char*, but now I am getting the following errors:
[Linker error] undefined reference to `int maxn<int>(int*, int)'
[Linker error] undefined reference to `double maxn<double>(double*, int)'

Your template function at the bottom returns void instead of Any?

Quote:

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Originally Posted by tabstop

Your template function at the bottom returns void instead of Any?

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I fixed it, and the code now compiles. Thanks.

Ok, I did not fix everything. How can I get the following function to return an address for the longest string int the array of char*:

Code:

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template <> char* maxn(char* ar[], int n) {
    using namespace std;
    char * temp = ar[0];
    for(int i = 1; i < n; i++) {
        if(strlen(temp) < strlen(ar[i])) {
            temp = ar[i]; } }
    return temp;
}

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In order to get your code to compile, I had to ditch the prototypes and move the function definitions up to the top (otherwise I was getting an error about specialization after instantiation). Doing so, I got the correct answer. I am not wise enough in the ways of template prototypes to say whether you can make what you have work.

STL itself in these cases provides two overloads: the second one takes a comparison function. Everything that can be compared with operator < works fine (including std::string). If you want more unusual comparisons or work with C-style strings you do more work and provide a comparison function.

In this particular case this approach might be more justified: you seem to want an unusual comparison for C style strings. Comparing them by length and not by the value of contents as you compare every other type seems so specific that it isn't justified as a specialization.