inconsistent string sorting with template method

Question

I am trying to do a template function that sorts 3 elements,
numbers(int, double) work fine but string's do not behave as expected

#include <cstdlib>
#include <algorithm>
#include <iostream>

using namespace std;

template<typename TYPE>
void sort3(TYPE n1, TYPE n2, TYPE n3) {
    TYPE arr[3];
    arr[0] = n1;
    arr[1] = n2;
    arr[2] = n3;
    sort(arr, arr + 3);

    for (int i = 0; i < 3; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;
}

int main(int argc, char** argv) {
    sort3("B", "Z", "A");
    sort3(10.2, 99.0, 1.9);
    sort3(200, 50, 1);
    return 0;
}

gives me the following output:

A Z B

1.9 10.2 99

1 50 200

to my understanding sort3("B", "Z", "A"); should give me A B Z
it is not OS specific since it gives me the same result in online compiler
what is happening there ?

Possible duplicate of: stackoverflow.com/questions/14131491/… — user2155932
– user2155932, Commented Apr 21, 2013 at 14:05

Andy Prowl · Accepted Answer · 2013-04-21 14:04:22Z

4

That's because what you are passing are string literals, which are arrays of characters. When passing them as arguments to a function, arrays decay to pointers. Therefore, what you are sorting is the value of those pointers (and not the strings they are pointing to).

Try this:

#include <string>

// ...

sort3(std::string("B"), std::string("Z"), std::string("A"));

This works because an overload of operator < for std::string exists, and std::sort uses operator < to compare elements when a custom comparator is not passed as a third argument (which is your case).

answered Apr 21, 2013 at 14:04

Andy Prowl

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4 Comments

Borian Over a year ago

can I create a generic comparator that I can pass "a" instead of string("a") ?

Andy Prowl Over a year ago

@Borian: Yes, you could do that. Just define a functor that has two call operators, one for const char* that would construct std::string from the operands and compare those objects, and another operator template that uses < for any type

hr_117 Over a year ago

@Andy Prowl: What do you think about a "specialised" (surly there is a more correct name for that) template function for const char *, calling the sort3 with strings?

Andy Prowl Over a year ago

@hr_117: You probably mean an overload of sort3 that accepts three const char* and calls the generic sort3 this way: sort3(std::string(arg1), std::string(arg2), std::string(arg3));. Yes, that is also a possibility :)

Vaughn Cato · Accepted Answer · 2013-04-21 14:04:02Z

3

You end up comparing the pointers and not the actual string contents. One way to solve it would be like this:

sort3(std::string("B"), std::string("Z"), std::string("A"));

answered Apr 21, 2013 at 14:04

Vaughn Cato

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Comments

hr_117 · Accepted Answer · 2013-04-21 14:54:05Z

1

IF you like to stay with calling sort3 with const char* you can add a "specialised" template:

template<>
void sort3(const char* n1, const char* n2,const char* n3) {
    sort3( string(n1), string(n2), string(n3));
}

With this sort3("B", "Z", "A"); also will work.

But your question "what is happening there ?" is already answered by Andy Prowl.

edited Apr 21, 2013 at 14:54

answered Apr 21, 2013 at 14:43

hr_117

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