Memset on vector C++

Question

Is there any equivalent function of memset for vectors in C++ ?

(Not clear() or erase() method, I want to retain the size of v

Answer 1

You can use assign method in vector:

Assigns new contents to the vector, replacing its current contents, and modifying its size accordingly(if you don't change vector size just pass vec.size() ).

For example:

vector<int> vec(10, 0);
for(auto item:vec)cout<<item<<" ";
cout<<endl;
// 0 0 0 0 0 0 0 0 0 0 

// memset all the value in vec to 1,  vec.size() so don't change vec size
vec.assign(vec.size(), 1); // set every value -> 1

for(auto item:vec)cout<<item<<" ";
cout<<endl;
// 1 1 1 1 1 1 1 1 1 1

Cited: http://www.cplusplus.com/reference/vector/vector/assign/

Answer 2

Another way, I think I saw it first in Meyers book:

// Swaps with a temporary.
vec.swap( std::vector<int>(vec.size(), 0) );

Its only drawback is that it makes a copy.

Answer 3

Use std::fill():

std::fill(myVector.begin(), myVector.end(), 0);

Answer 4

If your vector contains POD types, it is safe to use memset on it - the storage of a vector is guaranteed to be contiguous.

memset(&vec[0], 0, sizeof(vec[0]) * vec.size());

Edit: Sorry to throw an undefined term at you - POD stands for Plain Old Data, i.e. the types that were available in C and the structures built from them.

Edit again: As pointed out in the comments, even though bool is a simple data type, vector<bool> is an interesting exception and will fail miserably if you try to use memset on it. Adam Rosenfield's answer still works perfectly in that case.