Initialize Vector Reserved Vectors

Question

I have a vector of vectors:

std::vector<std::vector<T>> v;

I want to initialize this vector with 5 elements (5 T vectors). Each of these vectors will contain from 0 to 10 elements. Obviously, I need internal vectors reserved with 10, and not the size of 10. I do not need excessive redistribution or copies. In other words, I need an emplace construct.

Since it std::vectordoes not provide a constructor with the necessary number of elements for redundancy, I came up with this idea:

std::vector<std::vector<T>> v(5,
    [](){
            std::vector<T> temp;
            temp.reserve(10);
            return temp;
    }());

Questions:

• It's really? Does this behavior support undefined?
• Did I really minimize the resources needed to complete the steps above? I feel like there are unnecessary copies in my approach.

Answer 1

• , , . , undefined.
• , .

, , . capacity() v, , 10. , vector copy - .

, , reserve() , :

std::vector<std::vector<T>> v(5);
for(auto& vec : v) {
    vec.reserve(10);
}

, .