# How to do gc without gc --- Take C++ as a example ---- Usually we do things like this ```cpp int main() { int* A = new int; free(A); } ``` ---- Sometimes we delete it to early ```cpp int main() { int* A = new int; free(A); cout << *A << endl; } ``` > Undefine behavior ---- Sometimes we delete it to late ```cpp int main() { int* A = new int; cout << *A << endl; } ``` > Memory leak --- Is is possible we prevent it? ---- ## Reference counting ---- ```cpp= int main() { std::shared_ptr<int> p = std::make_shared<int>(2); cout << *p << endl; //2 *p = 5; cout << *p << endl; //5 } ``` ---- How it works? by reference counting Let's do a exam. ---- ```cpp= class test { public: test() {cout << "I burn!" << endl;} ~test() {cout << "I die!" << endl;} }; int main() { test* p = new test; cout << "end program" << endl; return 0; } ``` ---- ```cpp= I burn! end program ``` ---- ```cpp= class test { public: test() {cout << "I burn!" << endl;} ~test() {cout << "I die!" << endl;} }; int main() { std::shared_ptr<test> p = std::make_shared<test>(); cout << "end program" << endl; return 0; } ``` ---- ``` I burn! end program I die! ``` ---- ```cpp= class test { public: test() {cout << "I burn!" << endl;} ~test() {cout << "I die!" << endl;} }; int main() { { std::shared_ptr<test> p = std::make_shared<test>(); } cout << "end program" << endl; return 0; } ``` ---- ``` I burn! I die! end program ``` ---- ``` we have a obj, count = 0 when value = obj, count++ when value is destroy, count-- when count == 0, free(obj) ``` --- Let's try to do it by ourself ---- ``` https://code.sololearn.com/c2rR1h9G9ehg ```