I recently learned how to use the vecor library in C++. I don't understant what is the vector default constructor or when should I use it.
If for example I have a struct and want to create a vector of n structs. Is the following code valid:
struct item
{
string id;
string name;
};
vector <item> vitem;
vitem.push_back(item());
Does this vector have one element? or no elements at all?
Yes, this calls the default constructor. A default constructor is a constructor that takes no arguments. The default constructor of std::vector initializes the vector with zero elements.
std::vector<item> items; // implicitly calls default constructor
It has. The ctor is called immediately if you are declaring vector as var instead pointer.
If for example I have a struct and want to create a vector of n structs. Is the following code valid:
No, this code is invalid because it does not create a vector of n structures. It creates an empty vector. If you want to create a vector of n structures you should explicitly specify the number of created elements. For example
std::vector<item> vitem( n );
or you can at first create an empty vector and then resize it. For example
srd::vector<item> vitem;
vitem.resize( n );
Or you can use method assign
std::vector<item> vitem;
vitem.assign( n, item() );
std::cout << vitem.size() << std::endlholds the answer to your question.