我可以合法地写入常量向量指向的数据，例如对其进行排序吗？

Question

I have a constant integer vector const vector<int> v = {5,1,3,2,4} . How can I sort it without using any extra memory like vector, array etc.

Answer 1

You can't.

Your vector is immutable. It cannot be changed. There is no [legal] way to re-order its contents.

By extension, if you want a sorted version of its contents, you will need a new vector, which needs more memory.

Answer 2

As you have shown in your answer the solution is to cast away const from the vector, ie:

vector<int>&temp = const_cast<vector<int>&>(v);
sort(temp.begin(), temp.end());

This works because you are not changing the state of the vector. The only thing standard says about undefined behavior and casting away const is in [dcl.type.cv]/4

Any attempt to modify ( [expr.ass] , [expr.post.incr] , [expr.pre.incr] ) a const object ( [basic.type.qualifier] ) during its lifetime ( [basic.life] ) results in undefined behavior. [...]

Any nothing in [expr.ass], [expr.post.incr], and [expr.pre.incr] applies to this example so you are not considered to be modifying the object.

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I do feel this is a defect though. The order of the elements matter for comparison operators so this modification should apply.

Answer 3

I have tested using c++11. In my job, it's working fine for me

You can sort any constant container in c++ using a pointer.

In your case, if you want to sort this vector using sort(v.begin(),v.end()) then it will result in some runtime error due to violation of const .

But you can sort your container in following way-

vector<int>*temp=(vector<int>*)&v;
sort(temp->begin(),temp->end());

After this, your constant vector v will be like this -

const vector<int> v = {1,2,3,4,5}