In practice, the vector series in c ++ -- creating vector of local structure AND vector of structs initialization

In practice, the vector series in c ++ -- creating vector of local structure AND vector of structs initialization

I have never used it beforeVector Now, write a short code:

#include 
  
   #include 
   
    int main() {    struct st { int a; };    std::vector
    
      v;    v.resize(4);    for (std::vector
     
      ::size_type i = 0; i < v.size(); i++) {        v.operator[](i).a = i + 1; // v[i].a = i+1;    }    for (int i = 0; i < v.size(); i++)    {        std::cout << v[i].a << std::endl;    }}
     
    
   
  

Compiled successfully with VS2015. Running result:
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However, this is only allowed after C ++ 11. In the past, the compiler did not allow you to write such a syntax, and the local structure in the vector container was not allowed.

Furthermore, what if there are several fields in struct?

#include
  
   #include
   
    #include
    
     using namespace std;struct subject {    string name;    int marks;    int credits;};int main() {    vector
     
       sub;    //Push back new subject created with default constructor.    sub.push_back(subject());    //Vector now has 1 element @ index 0, so modify it.    sub[0].name = "english";    //Add a new element if you want another:    sub.push_back(subject());    //Modify its name and marks.    sub[1].name = "math";    sub[1].marks = 90;    sub.push_back({ "Sport", 70, 0 });    sub.resize(8);    //sub.emplace_back("Sport", 70, 0 );    for (int i = 0; i < sub.size(); i++)    {        std::cout << sub[i].name << std::endl;    }}
     
    
   
  

However, the above practice is not good. We should first construct the object and then perform push_back. It may be more wise.
Subject subObj;
SubObj. name = s1;
Sub. push_back (subObj );

This involves a question. Why don't we use emplace_back instead of push_back? This is the topic we will discuss next.