C++ function template using default parameters

Question

#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <functional>
using namespace std;

template <typename Object, typename Comparator>
const Object &findMax(const vector<Object> &arr,
         const Comparator &isLessThan = less<Object>())
{
    int maxIndex = 0;

    for (int i = 1; i < arr.size(); i++) {
        if (isLessThan(arr[maxIndex], arr[i])) {
            maxIndex = i;
        }
    }
    return arr[maxIndex];
}

int main()
{
    vector<string> arr(3);
    arr[0] = "ZED";
    arr[1] = "alli";
    arr[2] = "crocode";
//...
    cout << findMax(arr) << endl;
    return 0;
}

When I compile it with g++, it gives the following error:

test4.cpp: In function ‘int main()’:
test4.cpp:48:24: error: no matching function for call to ‘findMax(std::vector<std::basic_string<char> >&)’
test4.cpp:48:24: note: candidate is:
test4.cpp:10:15: note: template<class Object, class Comparator> const Object& findMax(const std::vector<Object>&, const Comparator&)

I can't figure out your error, but you could consider std::max_element — Neil Kirk
– Neil Kirk, Commented Sep 24, 2013 at 11:58
Possible duplicate of C++ template function default value. — Peter Mortensen
– Peter Mortensen, Commented Jun 23, 2015 at 21:46

Angew is no longer proud of SO · Accepted Answer · 2013-09-24 12:10:45Z

15

Template parameters cannot be deduced from default arguments. C++11, [temp.deduct.type]§5:

The non-deduced contexts are:

...

A template parameter used in the parameter type of a function parameter that has a default argument that is being used in the call for which argument deduction is being done.

...

You can get around this using overloading:

template <typename Object, typename Comparator>
const Object &findMax(const vector<Object> &arr, const Comparator &isLessThan)
{
    int maxIndex = 0;

    for (int i = 1; i < arr.size(); i++) {
        if (isLessThan(arr[maxIndex], arr[i])) {
            maxIndex = i;
        }
    }
    return arr[maxIndex];
}

template <typename Object>
const Object &findMax(const vector<Object> &arr)
{
    return findMax(arr, std::less<Object>());
}

edited Sep 24, 2013 at 12:10

answered Sep 24, 2013 at 12:00

Angew is no longer proud of SO

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4 Comments

tangwenqiang Over a year ago

when 'findMax' is a function member of a class template it works well.why?

Angew is no longer proud of SO Over a year ago

@tangwenqiang You mean it's a member function template? Or is it a plain member function and Comparator is a template parameter of the enclosing class template?

tangwenqiang Over a year ago

1 #include <iostream> 2 using namespace std; 3 template <typename Object> 4 //class A { 5 // public: 6 Object retself(const Object &initVal = Object()) 7 { 8 return initVal; 9 } 10 //}; 11 int main() 12 { 13 //A<int> test; 14 cout << retself() << endl; 15 return 0; 16 } run it(will get error); delete the '//', replace line 14 with 'cout << test.retself() << endl (will be ok); I want to konw why?

Angew is no longer proud of SO Over a year ago

@tangwenqiang Because when it's a non-template member function of a class template, the template parameter comes from the enclosing class template and does not have to be deduced. So no problem there.

David · Accepted Answer · 2013-09-24 12:04:31Z

5

Default the template argument and the function argument. Use max_element (actually, don't even define this function, just use max_element wherever you would have called this).

template <typename Object, typename Comparator = std::less<Object>>
const Object &findMax(const vector<Object> &arr, Comparator comp = Comparator())
{
    return *std::max_element(arr.cbegin(), arr.cend(), comp);
}

Disclaimer: not tested, and must have C++11

answered Sep 24, 2013 at 12:04

David

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Comments

Peter Mortensen · Accepted Answer · 2015-06-23 21:48:39Z

4

Using default template parameters in C++11, your function could be written this way:

template <typename Object, typename Comparator = std::less<Object> >
const Object &findMax(const vector<Object> &arr, const Comparator isLessThan = Comparator())
{
    int maxIndex = 0;

    for (int i = 1; i < arr.size(); i++) {
        if (isLessThan(arr[maxIndex], arr[i])) {
            maxIndex = i;
        }
    }
    return arr[maxIndex];
}

Note the usage of default template parameter typename Comparator = std::less<Object>.

edited Jun 23, 2015 at 21:48

Peter Mortensen

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answered Sep 24, 2013 at 12:03

Oragon Efreet

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