c++ vector constructor instantiation conflicts

Question

I'm trying to implement vector container refering to http://www.cplusplus.com/reference/vector/vector/vector/ . There are two constructors causing some problems:

template <typename T>
vector (size_type n, const T& val);

template <typename T>
template <typename InputIterator>
vector (InputIterator first, InputIterator last);

When I use vector<int> vec(100, 1); . It seems that both template constructor make instantiations. So I got template <typename T> vector (size_type n, const T& val) with T=int and template <typename T> template <typename InputIterator> vector (InputIterator first, InputIterator last) with T=int, InputIterator=int .

How do I solve that?

Answer 1

Note that the standard (since C++11) requires that the overloaded constructor of std::vector taking iterator as parameter only participates in overload resolution when the arguments passed act as iterator.

This overload only participates in overload resolution if InputIt satisfies InputIterator , to avoid ambiguity with the overload (2).

You can use SFINAE with std::iterator_traits to accomplish it for your own implementation of vector. eg

template <typename InputIterator, typename = typename std::iterator_traits<InputIterator>::value_type>
vector (InputIterator first, InputIterator last) { ... }

For the case of InputIterator = int , std::iterator_traits<int> doesn't contain any member types like value_type , then the overload will be ruled out of overload resolution.

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BTW: More precisely, the problem is not "both template constructor make instantiations"; I'm not sure what's the exact type of size_type in your implementation, if it's some type other that int , like std::size_t , then for vector<int> vec(100, 1); the 1st overload won't be selected because it requires to convert the 1st argument from int to std::size_t , then the 2nd overload will be selected because it could produce a perfect match instantiation with InputIterator = int . That's not the behavior we expect.