问题
For C++11, is there still a performance difference between the following?
(for std::map<Foo, std::vector<Bar> >
as an example)
map[key] = myVector and map.emplace(key, myVector)
The part I'm not figuring out is the exact internal of operator[]. My understanding so far has been (when key doesn't exist):
- Create a new key and the associated empty default vector in place inside the map
- Return the reference of the associated empty vector
- Assign myVector to the reference???
The point 3 is the part I couldn't understand, how can you assign a new value to a reference in the first place?
Though I cannot sort through point 3 I think somehow there's just a copy/move required. Assuming C++11 will be smart enough to know it's gonna be a move operation, is this whole "[]" assignment then already cheaper than insert()? Is it almost equivalent to emplace()? ---- default construction and move content over, versus construct vector with content directly in place?
回答1:
There are a lot of differences between the two.
If you use operator[]
, then the map
will default construct the value. The return value from operator[]
will be this default constructed object, which will then use operator=
to assign to it.
If you use emplace
, the map
will directly construct the value with the parameters you provide.
So the operator[]
method will always use two-stage construction. If the default constructor is slow, or if copy/move construction is faster than copy/move assignment, then it could be problematic.
However, emplace
will not replace the value if the provided key already exists. Whereas operator[]
followed by operator=
will always replace the value, whether there was one there or not.
There are other differences too. If copying/moving throws, emplace
guarantees that the map
will not be changed. By contrast, operator[]
will always insert a default constructed element. So if the later copy/move assignment fails, then the map
has already been changed. That key will exist with a default constructed value_type
.
Really, performance is not the first thing you should be thinking about when deciding which one to use. You need to focus first on whether it has the desired behavior.
C++17 will provide insert_or_assign, which has the effect of map[] = v;
, but with the exception safety of insert/emplace
.
how can you assign a new value to a reference in the first place?
It's fundamentally no different from assigning to any non-const
reference:
int i = 5;
int &j = i;
j = 30;
i == 30; //This is true.
来源:https://stackoverflow.com/questions/39176770/is-c11-operator-equivalent-to-emplace-on-map-insertion