Cant understand how this code works....

[gpraghuram]

HI all,
I wrote the following piece of code

class A
{
    A(){cout<<"ctor"<<endl;
    ~A(){cout<<"dtor"<<endl;
};
 
int main()
{
    std::map<A> a_map(10);
    cout<<"size is:"<<10<<endl;
    return 0;
}
 

the output is
ctor
dtor
10
dtor
dtor
dtor
dtor
dtor
dtor
dtor
dtor
dtor
dtor

The destructor is called 10 times but the constructor is called only once.
I tried this in Linux using g++.

Can somebody throw some light on this.

Thanks
Raghuram

[Banfa]

This is rather dependent on how std::map is implemented on your system however here is my guess

the distructor is actually called 11 times (once before your output line) and the default constructor once.

I guess that the constructor of std:map constructs a single A (calling the default constructor once) then copies it to it's entries (calling the copy constuctor 10 times) then deletes the temporary A object calling the destructor once.

Then you output line runs, then the map is deleted and each item in the map is deleted calling the destructor.

You could implement a copy constructor to test this theory.

[Ganon11]

Doesn't a map disallow duplicates? Although I guess that, since A doesn't provide an overridden operator== function, the map will compare based on addresses or some other such comparison, preventing it from seeing these as duplicates.

[weaknessforcats]

Your map needs to have pair<> objects.

The first member or the pair is the key and the second member is a A object.

map<int, A>  themap;
 

By the way, your code won't compile. If it does, then you have enabled C++ language extenssions that are non-standard.

[gpraghuram]

Your map needs to have pair<> objects.

The first member or the pair is the key and the second member is a A object.

map<int, A>  themap;
 

By the way, your code won't compile. If it does, then you have enabled C++ language extenssions that are non-standard.

I have made a mistake.
It is a vector and not a map.
I got some idea from Banfa, anyway i will try with copy constructor and post back again.

Raghuram

[gpraghuram]

I have made a mistake.
It is a vector and not a map.
I got some idea from Banfa, anyway i will try with copy constructor and post back again.

Raghuram

Banfa, u are right.
As u say the copy constructor is being called 10 times and due to which the destructor is being called 10 times.
Thanks for your explanation.

Raghuram