Vector iterator as a function parameter: what's happening in this code?

Asked Oct 15 '17 at 18:22

Active Oct 15 '17 at 19:43

Viewed 24 times

I'd like to know why is this code legal:

std::vector<int>::iterator & foo(std::vector<int>::iterator & iter) {
    return ++iter;
}

int main() {
    std::vector<int> v = { 1,2,3,4 };

    std::cout << *foo(v.begin()) << std::endl;
}

The function accept a non-const reference to an iterator, but I have not initialized any variables holding that iterator, I simply passed the result of v.begin() as a reference, but the function is able to increase its value, return it, and use the result to print an element of the vector:

Where is the iterator stored? I'm still learning C++ (Started around 3 weeks ago) so I'm a bit confused, but I have an "idea":

Is a temporary variable initialized just before the call, like when passing a literal to a function that accept a const reference (Example below)?

std::vector<int>::iterator & foo(std::vector<int>::iterator & iter) {
    return ++iter;
}

int main() {
    std::vector<int> v = { 1,2,3,4 };

    std::vector<int>::iterator tempIter = v.begin();
    std::cout << *foo(tempIter) << std::endl;
}

Thanks!

edited Oct 15 '17 at 19:43

asked Oct 15 '17 at 18:22

Frax

Uh... `std::vector::iterator tempIter = v.begin();`? – Passer By Oct 15 '17 at 18:36
That is my theory, I think it creates that hidden variables to store the iterator, the real code is the first block – Frax Oct 15 '17 at 19:21
Which compiler do you use? Both GCC and Clang are giving me an error. – Revolver_Ocelot Oct 15 '17 at 19:53
And Visual Studio 2017 does not accept this code either. – Revolver_Ocelot Oct 15 '17 at 19:55
I'm on Windows 10 and using the last version of Visual Studio 2017, and the output is correct, 2... – Frax Oct 15 '17 at 19:59
1) Turn on warnings 2) Turn on strict standard conformace. Check if this behavior persist – Revolver_Ocelot Oct 15 '17 at 20:03
Can you tell me how to turn on warnings and strict standard conformace? – Frax Oct 15 '17 at 20:05
Project - properties - C++ - General - Warning Level - Select Level4 then C++ - Language - Disable Language Extentions - Select Yes – Revolver_Ocelot Oct 15 '17 at 20:11

Vector iterator as a function parameter: what's happening in this code?

0 Answers0