Question

I have a method to find number in range of iterator like below. I want it has reference parameter and return reference of iterator.

// find number in range of begin and end, if number found return reference of iterator, throw exception otherwise
vector<int>::iterator &find_int(vector<int>::iterator &begin, vector<int>::iterator &end, int &number)
{
    for (; begin != end; ++begin)
    {
        if (*begin == number)
        {
            return begin;
        }
    }

    throw "not found";
}

I want to call this method as following code but it can’t compile.

vector<int>::iterator i5 = find_int(vi1.begin(), vi1.end(), 5);

I want to know how can I call find_int method ?

Update
Error message when compile the code

C:\Windows\system32\cmd.exe /c g++ c:\cpp\ch9.cpp -o c:\cpp\ch9.exe
c:\cpp\ch9.cpp: In function 'int main()':
c:\cpp\ch9.cpp:41:63: error: invalid initialization of non-const reference of ty
pe 'std::vector<int>::iterator& {aka __gnu_cxx::__normal_iterator<int*, std::vec
tor<int> >&}' from an rvalue of type 'std::vector<int>::iterator {aka __gnu_cxx:
:__normal_iterator<int*, std::vector<int> >}'
c:\cpp\ch9.cpp:30:24: error: in passing argument 1 of 'std::vector<int>::iterato
r& find_int(std::vector<int>::iterator&, std::vector<int>::iterator&, int&)'
shell returned 1
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Answer 1

That function really shouldn’t be taking iterator&‘s (or the int&¹) anyway. It should take the arguments by value, and return the iterator by value.

Even if you didn’t get a compiler error because of the int literal you’re trying to pass it (5) where it expects an int&¹, the reason you can’t call it the way you’re doing is because you would be passing a reference to a temporary value, then getting a reference to that temporary through the return value of the function, and by the next line of code, the reference would refer to a destroyed object.

You can call it like this:

vector<int>::iterator beg = vi1.begin(), end = vi1.end();

vector<int>::iterator& pos = find_int(beg, end, 5);

Note that &pos == &beg.

However, if this isn’t a didactic exercise, I would recommend to do what shuttle87 suggested, which is to use std::find.

---

¹ Thanks Ghita for pointing this out.