The advantages of saving pointers when saving objects with vector and the use of reserve

  Code 1

#include <vector> #include <stdio.h> class A { public: A() { printf("A()/n"); } ~A() { printf("~A()/n"); } A(const A& other) { printf("other/n"); } }; int main() { A a; A b(a); A c = a; return 0; }

  Execution results 1

A() other other ~A() ~A() ~A()

  Code 2

#include <vector> #include <stdio.h> class A { public: A() { printf("A()/n"); } ~A() { printf("~A()/n"); } A(const A& other) { printf("other/n"); } }; int main() { A a; A b(a); A c = a; printf("----------/n"); std::vector<A> vec; //vec.reserve(3); vec.push_back(a); vec.push_back(b); vec.push_back(c); return 0; }

  result 2

A() other other ---------- other other ~A() other other other ~A() ~A() other other other other ~A() ~A() ~A() ~A() ~A() ~A() ~A() ~A() ~A()

  Code 2 Commented out vec.reserve(3) open , result 3

A() other other ---------- other other other ~A() ~A() ~A() ~A() ~A() ~A()

 

  The instructions are in use vector when , What is inserted is a copy of the object to be inserted , If vector When the class object in is large , Will affect performance , There are also some deep and shallow copy problems when using copy construction , In addition, through the results 2 And results 3 We can know when vector When there is not enough space to apply for , It will apply for space again and may give up the original space , In this way, the number of copy constructions called is more , So we're using vector It should be passed through its member function reserve Apply in advance for a value we estimate , You can rest assured , When reserve When the space is not enough , vector Will still automatically apply for space

  Here's how to use vector The method of storing class pointers in , Be sure to insert vector The life cycle of the object pointer pointing to the content in , In addition, if it is new Coming out , If not elsewhere delete It should be traversed in due course vector Find out delete

#include <vector> #include <stdio.h> class A { public: A() { printf("A()/n"); } ~A() { printf("~A()/n"); } A(const A& other) { printf("other/n"); } }; int main() { A *a = new A; A *b = new A(*a); A *c = new A(*a); printf("----------/n"); std::vector<A*> vec; vec.reserve(3); vec.push_back(a); vec.push_back(b); vec.push_back(c); std::vector<A*>::iterator iter = vec.begin(); for (; iter!=vec.end(); ++iter) { delete *iter; //*iter = a , b, c } vec.clear(); return 0; }

  result

A() other other ---------- ~A() ~A() ~A()