Why does my destructor appear to be called more often than the constructor?

Question

#include
using namespace std;

class A{
public:
    static int cnt;
    A()
    { 
        ++cnt; 
        cout<<\"constructor:\"<

        

Answer 1

You need to add a copy constructor that increases the counter.

A(const A&)
{ 
    ++cnt; 
    cout<<"copy constructor:"<

If you don't add it explicitly, the compiler generates one that does nothing with the counter cnt.

This expression

A a1 = f(a0);

is creating copies of a0, which make use of the copy constructor. The exact number of copies may vary depending on copy elision, but your cnt should be 0 at the end of the program.

Note: In C++11, you should also consider the possibility of a compiler generated move copy constructor, however, once you declare your own copy constructor, the compiler no longer generates the move version.