Get index of object inserted into a vector

问题

How can I get the position where my object was actually inserted?

#include <vector>
using namespace std;

vector<SomeClass> list;
SomeClass object;
list.push_back(object);

list[...].method(); // I do not have the key

Unfortunately push_back does not return anything since its return type is void.

回答1:

If v is your vector, the following will give you the position (that is, the index):

v.push_back(object);
size_t pos = v.size() - 1;

Or you can look at the size() before calling push_back(). Then you won't need to subtract one.

回答2:

You can use the back() member to obtain a reference to the last element:

list.push_back(object);
list.back();

Or, since push_back() simply adds the object to the end, the index of the newly-inserted element is the vector size minus one:

list.push_back(object);
vector<my_class>::size_type object_pos = list.size() - 1;

回答3:

If you need to locate a particular element after you have them, and you don't want to save the index ahead of time (in case you do something to the vector like sort/add/remove/etc), you could also use the find algorithm.

回答4:

While NPE's correct answer shows a pragmatic POV, I concentrate on the reasoning for this simple answer. The observable behaviour is just the purpose of the push_back member function:

Appends the given element value to the end of the container.

So the index is always equal to v.size() before insertion or, equivalently, v.size()-1 after insertion. If this assertion cannot been fulfilled, the capacity of the container could not be grown or your class throws an exception in the copy/move constructor (see section Exceptions in the documentation).

来源：https://stackoverflow.com/questions/13879484/get-index-of-object-inserted-into-a-vector