sizeof() a vector

问题:

I have a vector<set<char> > data structure (transactions database) and I want to know the size of it. When I use sizeof() with each set<char> the size is 24 in spite of the set contains 3, 4 or 5 chars. Later, when I use sizeof() with the vector<set<char> > the size is 12... I suppose this is not the way to know the size of a data structure. Any help? Thanks.

回答1:

You want vector::size() and set::size().

Assuming v is your vector, do this:

size_t size = 0;
for (vector<set<char> >::const_iterator cit = v.begin(); cit != v.end(); ++cit) {
    size += cit->size();
}

sizeof() is giving you the in-memory size of the object/type it is applied to, in multiples of sizeof(char) (usually one byte). If you want to know the in-memory size of the container and its elements, you could do this:

sizeof(v) + sizeof(T) * v.capacity(); // where T is the element type

回答2:

sizeof returns size of object itself. if it contains pointer to array for example it will not count size of array, it will count only size of pointer (4 on 32 bits) for vector use .size

回答3:

Vector is implemented using internal pointers to the actual storage. Hence sizeof() will always return the same result which does not include the data storage itself. Try using the vector::size() method instead. This will return the number of elements in the vector.

回答4:

sizeof() is computed at compile time, so there is no way it can tell you how many elements it has inside.

Use the size() method of the vector object.

回答5:

vector in STL is a class template, when you give template parameter inside <SomeType> following vector, C++ compiler generated code for a class of type SomeType. So when you populate the vector using push_back, you are actually inserting another object of SomeType so when you request .size() from the compiler it gives you the number of SomeType objects inserted by you.
Hope that helps!

回答6:

Use vector::size() member function to find out number of items in the vector. Hint - they are allocated on the free store.