C ++ std :: vector清除所有元素

Question

Consider a std::vector :

std::vector<int> vec;
vec.push_back(1);
vec.push_back(2);

Would vec.clear() and vec = std::vector<int>() do the same job? What about the deallocation in second case?

Answer 1

vec.clear() clears all elements from the vector, leaving you with a guarantee of vec.size() == 0 .

vec = std::vector<int>() calls the copy/move(Since C++11) assignment operator , this replaces the contents of vec with that of other . other in this case is a newly constructed empty vector<int> which means that it's the same effect as vec.clear(); . The only difference is that clear() doesn't affect the vector's capacity while re-assigning does, it resets it.

The old elements are deallocated properly just as they would with clear() .

Note that vec.clear() is always as fast and without the optimizer doing it's work most likely faster than constructing a new vector and assigning it to vec .

Answer 2

They are different:

clear is guaranteed to not change capacity.

Move assignment is not guaranteed to change capacity to zero, but it may and will in a typical implementation.

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The clear guarantee is by this rule:

No reallocation shall take place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the value of capacity()

Post conditions of clear :

Erases all elements in the container. Post: a.empty() returns true

Post condition of assignment:

a = rv;

a shall be equal to the value that rv had before this assignment

a = il;

Assigns the range [il.begin(),il.end()) into a. All existing elements of a are either assigned to or destroyed.