initializer_ List tool library learning

initializer_list Is the initialization list class , And vector,list,map Such as compared to , Using it directly is less , But it is used in the initialization of many classes or templates , Because it is an initialization list , So it means , Yes initializer_list Can't add , Delete , Change , It's equivalent to const Datalist , Therefore, it provides relatively few functions , Only size(), begin(), end(), Nonmember functions std::begin(std::initializer_list),std::end(std::initializer_list),

 stay c++11 Before , If we want to be right vector To initialize 5 Elements , We need this 
vector<int> vec;
vec.push_back(1);
vec.push_back(2);
vec.push_back(3);
vec.push_back(4);
vec.push_back(5);
C++11 in the future , I can directly use the following code 
vector<int> vec = {1,2,3,4,5};
 Why can it be like this , original vector Constructor reference of , There is one initializer_list<T> Parameters of , Here is VS2017 vector The source code in the header file 

  It's not just vector, list, map, set, forward_list Constructors such as support list initialization

Here is list Constructor functions initializer_list Parameters

  Here is map Constructor functions initializer_list Parameters

Here is an example of its use ：

#include <iostream>
#include <string>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <forward_list>
#include <initializer_list>

using namespace std;

int main()
{
    vector<int> vec = {1,2,3,4,5};
    cout << "vector ================== ";
    for(auto v: vec)
    {
        cout << v << "\t";
    }
    cout << endl;

    list<int> lis = {1,2,3,4,5};
    cout << "list ==================== ";
    for(auto l: lis)
    {
        cout << l << "\t";
    }
    cout << endl;

    map<int, string> ma = {
   {1, "first"}, {2, "second"}, {3, "three"}};
    cout << "map ===================== ";
    for(auto m: ma)
    {
        cout << m.first << "==>" << m.second << "\t";
    }
    cout << endl;

    initializer_list<int> init_list = {1,2,3,4,5,6,7,8,9};
    cout << "initializer_list ======== ";
    for(auto init_: init_list)
    {
        cout << init_ << "\t";
    }
    cout << endl;
    //size return initializer_list The total number of elements 
    cout << "init_list.size=========== " << init_list.size() << endl;
    //begin Point to initializer_list First element 
    cout << "init_list.first========== " << *init_list.begin() << endl;
    //end Point to initializer_list The next position of the last element 
    initializer_list<int>::iterator end1 = init_list.end();
    end1--;
    cout << "init_list.last=========== " << *end1 << endl;
    //std::begin Point to initializer_list First element 
    cout << "std::begin(init_list)==== " << *std::begin(init_list) << endl;
    //std::end Point to initializer_list The next position of the last element 
    initializer_list<int>::iterator stdEnd = std::end(init_list);
    --stdEnd;
    cout << "std::end(init_list)====== " << *stdEnd << endl;
    //*stdEnd = 50;  // Constant iterator returned , You can't change its value 

    cout << "Hello World!" << endl;
    return 0;
}

Running results ：

 initializer_list Use in functions and classes ：

#include <iostream>
#include <vector>
#include <initializer_list>

using namespace std;

template<class T>
struct InitList{
    std::vector<T> vec;
    InitList()// Default constructor 
    {
        cout << "default constructor==============" << endl;
        vec.clear();
    }
    InitList(T param1, T param2)// Constructor with parameters 
    {
        cout << "parametric constructor  ==============" << endl;
        vec.push_back(param1);
        vec.push_back(param2);
    }

    InitList(initializer_list<T> li)// Initialize the list constructor 
        :vec(li)
    {
        cout << "constructed with a " << li.size() << " -element list \n";
    }

    void append(initializer_list<T> li) // Function with list argument 
    {
        vec.insert(vec.end(), li.begin(), li.end());
    }
    std::pair<const T*, std::size_t> c_arr() const {
        return {&vec[0], vec.size()};// stay return  Statement, copy list initialization , It doesn't use std::initializer_list
    }
};

template <typename T>
void templated_fn(T) { cout << "call templated_fn ======== " << endl; }

int main()
{
    InitList<int> init1;

    InitList<int> init2(20, 31);

    InitList<int> init3 = {23,34,56,76,98};

    init3.append({82, 15, 39});
    cout << "the vector size is now " << init3.c_arr().second << " ints:\n";

    for(auto n: init3.vec)
        cout << n << "  ";
    cout << endl;
    cout << "Range-for over brace-init-list:\n";
    for(int x: {-10, -15, -18}) //auto  The rules of the law make this band for Work 
        cout << x << "  ";
    cout << endl;
    auto alint = {13, 15, 18};//auto The special rules of 
    cout << "the list bound to auto has size() = " << alint.size() << endl;
    //templated_fn({11,22,33}); // Compile error   {11,22,33}  It's not an expression , It has no type , so T It's impossible to deduce 
    templated_fn<initializer_list<int>>({11,22,33}); //OK
    templated_fn<vector<int>>({11,22,33}); // also OK

    cout << "Hello World!" << endl;
    return 0;
}

Running results ：

 

Reference resources ：

https://zh.cppreference.com/w/cpp/header/initializer_list