Erasing values from vector/map

Question

Is this a safe way of deleting all entries from my vector or map

Vector

my_vector.erase(my_vector.begin(),my_vector.end());

For example for a map

my_map.erase(my_map.begin(),my_map.end());

The map or vector structure contains elements whose de-allocation is taken care in the destructor of those elements

Does the iterator value returned by end() become invalid as it starts to erase elements?

Answer 1

Both erase() methods are designed to work for iterator ranges excluding the second one.

// erase elements in range [a,b)
Iterator erase(Iterator a, Iterator b);

So it is safe to call erase() as you do, but you might as well call clear() in both cases.

Answer 2

It's safe to call erase(begin, end) for std::vector/std::map , it also valid to other STL containers( list, set, deque etc ) which provide erase member function and iterator to iterate through elements.

As long as you pass in valid range(beg,end), below two ranges are also valid, erase takes no efforts:

c.erase(c.begin(), c.begin());
c.erase(c.end(), clend());

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std::vector::erase(beg,end) Removes all elements of the range [beg,end) and returns the position of the next element.

std::map::erase(beg,end) Removes all elements of the range [beg,end) and returns the following position (returned nothing before C++11).

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In STL internal implementation, it calls erase(begin,end) in a few functions like:

void clear() noexcept;
Effects: Behaves as if the function calls:
          erase(begin(), end());

void assign(size_type n, const T& t);
Effects:
   erase(begin(), end());
   insert(begin(), first, last);

As you can see, erase(begin(),end()); is the same as clear() .

Alternatively, you could call swap to clear a STL container which is suggested in More Effective STL :

vector<Contestant> v;
vector<Contestant>().swap(v); //clear v and minimize its capacity