CODEWITHSUNDEEP
c++iteratorconstantsauto
z_chong
z_chong

Reputation: 39

What has happened to this statement " auto iter = first"

What has happened to this statement:

auto iter = first;

Why iter++ can work well, as we all know, first is const&,

template<typename iteratorT, typename valueT>
iteratorT find(const iteratorT& first, const iteratorT& last,const valueT& value)
{
    auto iter = first;
    while(iter != last && *iter != value) iter++;
    return iter;
}

Why?

Upvotes: 1

Views: 164

Answers (1)

skypjack
skypjack

Reputation: 50550

That's how type deduction works when using auto, in the example you are getting a copy of first and that copy has type iteratorT.

You can use instead:

  • const iteratorT &iter = first
  • const auto &iter = first (or just auto &)
  • decltype(first) iter = first
  • decltype(auto) iter = first

All of them if you want iter to be a const reference, of course.

Thanks to @songyuanyao and @LogicStuff for theirs suggestions in the comments.

Consider also the following example that reproduces your issue in a cleaner way:

#include <type_traits>

int main() {
    int i = 42;
    const int & j = i;
    auto k = j;

    static_assert(std::is_same<decltype(k), int>::value, "!");
    static_assert(not std::is_same<decltype(k), const int &>::value, "!");
}

Upvotes: 5

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