STL algorithms with templated functions as parameter

Question

How is it possible to use a templated functions with the STL provided algorithms in <algorithm>? For example, this code does not compile because compiler can't deduce template parameters for the predicate function:

#include <iostream>
#include <algorithm>

template< typename CharType >
bool predicate( const CharType& c )
{
    return c == '0';
}

std::string
process_string( const std::string& str )
{
    std::string result;
    std::copy_if( str.begin( ),
                  str.end( ),
                  std::back_inserter( result ),
                  predicate );
    return result;
}

int main()
{
    std::cout << process_string("AK0NNDK0ASDAS0") << std::endl;
    return 0;
}

Jarod42 · Accepted Answer · 2016-04-22 11:58:19Z

6

Several ways: including

provide explicitly the type
```
predicate<std::string::value_type>
```
use lambda
```
[](auto&&e) { predicate(e); }
```

edited Apr 22, 2016 at 11:58

answered Apr 22, 2016 at 11:38

Jarod42

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2 Comments

Yakk - Adam Nevraumont Over a year ago

auto&& is more generic, but not required in this particular case.

stefaanv Over a year ago

Unfortunately, the lambda code doesn't work in c++11, which is a tag in the question. generic lambda's do work from c++14 on.

stefaanv · Accepted Answer · 2016-04-22 11:35:07Z

5

You can provide the type: predicate<std::string::value_type>

answered Apr 22, 2016 at 11:35

stefaanv

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Comments

score 1 · Accepted Answer · 2016-04-22 11:57:48Z

1

A template itself can not be a function argument. In this case you want to pass a function pointer to the template function, which you have to instantiate as predicate<char>:

std::copy_if( str.begin( ),
              str.end( ),
              std::back_inserter( result ),
              predicate<char> );

edited Apr 22, 2016 at 11:57

answered Apr 22, 2016 at 11:43

user2249683

Comments

Brian Rodriguez · Accepted Answer · 2016-04-27 08:04:54Z

1

Use an instantiated functor w/ a templated operator():

namespace detail {

struct predicateFunctor {
    template<typename CharType>
    bool operator()(const CharType& c)
    {
        return c == '0';
    }
};

} /*namespace detail*/

static auto predicate = detail::predicateFunctor{};

std::string process_string(const std::string& str)
{
    std::string result;
    std::copy_if(str.begin(), str.end(), std::back_inserter(result), predicate);
    return result;
}


int main()
{
    std::cout << process_string("AK0NNDK0ASDAS0") << std::endl;
    return 0;
}

Demo

edited Apr 27, 2016 at 8:04

answered Apr 22, 2016 at 11:45

Brian Rodriguez

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Comments

fghj · Accepted Answer · 2016-04-22 11:36:44Z

0

use static_cast to resolve overload issue:

std::copy_if( str.begin( ),
              str.end( ),
              std::back_inserter( result ),
              static_cast<bool (*)(const char&)>(predicate) );

answered Apr 22, 2016 at 11:36

fghj

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