CODEWITHSUNDEEP
c++templatesgenerics
zwjason
zwjason

Reputation: 1

C++ template argument inference

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

template<class InputIterator>
typename iterator_traits<InputIterator>::value_type
sumRange(InputIterator first, InputIterator last) {
    // ignore validate [start, last)
    typedef typename __iterator_traits<InputIterator>::value_type value_type;
    value_type sum = *first++;
    while (first != last) {
        sum = sum + *first++;
    }
    return sum;
}

int main() {
    array<int, 3> a{4, 5, 6};
    cout << sumRange(a.begin(), a.end()) << endl;
    return 0;
}

why the function sumRange can not apply to built-in type such as int* ?

But the STL algorithm accumulate work.

The error information is:

main.cpp:29:67: error: no type named 'value_type' in 'struct std::__iterator_traits<int*, void>'
     typedef typename __iterator_traits<InputIterator>::value_type value_type;

Upvotes: 0

Views: 49

Answers (1)

lorro
lorro

Reputation: 10880

For built-in types such as int[], you need to use std::begin(a) and std::end(a) on these types, also - as NathanOliver wrote - std::iterator_traits<> instead of the implementation detail __iterator_traits<>.

std::begin() and std::end() works for std containers as well (in which case, it delegates to the member functions begin() and end(), respectively), so these are more generic and thus preferred. It's also what range-based for uses.

Upvotes: 2

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