Add operator<< to std::vector

Question

I want to add operator<< to std::vector<string>. This is the operator

std::vector<std::string>& operator<<(std::vector<std::string>& op, std::string str) {
    op.push_back(str);
    return op;
}

Now I can add elements to vector this way:

std::vector<std::string> vec;
vec << "aaa" << "bbb" << "ccc";

But the following code isn’t compiled since cant pass temporary object by reference.

std::vector<std::string>() << "AAA" << "BBB";

How can I change the code to add elements to temporary vectors too?

I have a function which accepts const std::vector<std::string>& argument so I want to pass to it a vector in one line.

Answer 1

You can't. The only way it could be achieved would be if your operator was a member of the vector. (For some reason member functions can be invoked on temporaries).

As a nonmember function though what you're after is impossible

Also it's not a very good idea to overload operators for STL types such as std::vector. I think what you're trying to accomplish is already present in boost assign library. I don't believe you can write it better than they have, so better use boost if you need this.

Answer 2

I haven't actually thought about the consequences of this properly, but it seems you could have an overload for rvalues:

std::vector<std::string> operator<<(std::vector<std::string>&& op, std::string str) {
  op.push_back(std::move(str));
  return op;
}

Yes, it returns by value, but it will be optimized to a move due to §12.8/32:

When the criteria for elision of a copy operation are met or would be met save for the fact that the source object is a function parameter, and the object to be copied is designated by an lvalue, overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue.