How do I declare a vector of vector::size_type?

Question

I want a vector whose elements are of the type vector::size_type

However, you cannot declare:

vector<vector::size_type> aVec;

because size_type is part of the template itself, so you have to use the type itself, I need something like:

vector<vector<T>::size_type> aVec;

but what should the T be? It's really a circular problem. :)

If vector had just used size_t as its size type (but not had a special typedef for the size_type which could vary maybe based on the type that the vector is holding), I could just do:

vector<size_t> aVec;

but, that isn't the case. I suspect there is a valid reason for it being allowed to vary, but it is making this hard by having it part of the templated vector class instead of outside of it.

Thoughts?

Answer 1

std::vector<>::size_type是std::size_t类型的静态成员类型，所以你应该使用std::vector<std::size_t> vec是安全的

Answer 2

Why don't you use vector<vector<int>::size_type> myVector; ? If you never use the type vector<int> anywhere else, it doesn't cost anything as template instanciation is somehow lazy. You can use any other type than int if you which.

It is very unlikely that vector<T>::size_type will be different than vector<T'>::size_type .

Answer 3

(I am rewritting an answer here, without making any claims about the standard this time.)

You can make your best guess for the type (which can be std::size_t or std::vector<void*>::size_type ) and check after the fact.

std::vector<std::size_t> Avec;
static_assert(std::is_same<decltype(Avec)::size_type, decltype(Avec)::value_type>(), "bad guess");