vector::begin()的迭代器返回疑问：实际返回值究竟是什么？

Hey there! Let's break down exactly what vector::begin() returns, since it's easy to mix up what you see in debuggers vs. the actual return type.

First off, let's clear up the big misunderstanding: vector::begin() does NOT return 0, or any numeric value. It returns an iterator object—think of this as a "smart pointer" designed to point to elements inside the vector.

What's an iterator, exactly? #

For a std::vector<int>, the iterator returned by begin() is of type std::vector<int>::iterator (or const_iterator if the vector is const). Under the hood, compilers have their own implementations of this standard type—Visual Studio uses std::_Vector_iterator, which is why you see that in the "Type" column of your watch window. This type wraps a pointer to the vector's first element, but it's a full object with methods and operators that let you interact with the vector's elements safely.

Why does the debugger show the first element's value? #

The Visual Studio debugger is trying to be helpful here! When you watch v.begin(), it recognizes that this is a vector iterator and automatically dereferences it for you—so instead of showing you the iterator object itself (like the underlying pointer or internal members), it displays the value of the element the iterator is pointing to. If you want to see the iterator's actual internal details, you can expand the iterator entry in the watch window to view things like the underlying pointer (_Ptr in VS's implementation).

A quick code example to make this concrete #

#include <vector>
#include <iostream>

int main() {
    std::vector<int> v = {42, 88, 100};
    
    // Get the begin iterator: type is std::vector<int>::iterator
    auto it = v.begin();
    
    // Dereference the iterator to get the first element's value
    std::cout << "Value of first element: " << *it << "\n"; // Prints 42
    
    // Get the address of the first element (via the iterator)
    std::cout << "Address of first element: " << &(*it) << "\n";
    
    // Compare to the vector's raw data pointer
    std::cout << "Vector's data pointer: " << v.data() << "\n"; // Same address as above
    
    return 0;
}

Key takeaways #

• vector::begin() returns an iterator object, not a number or raw pointer (though iterators often behave like pointers for contiguous containers like vector).
• The debugger's watch display is simplifying things by showing the dereferenced value—always check the "Type" column to see the actual return type.
• The specific type like std::_Vector_iterator is just the compiler's implementation of the standard std::vector<T>::iterator interface.

内容的提问来源于stack exchange，提问作者W.Joe