Why member string acts different from member int

[volang]

struct a {

   string c;

   a(string x) {
       c = x;
   }
   
};


int main()
{   

   map <int, a*> m;
   m[1] = &a("hej");
   cout << m[1]->c;

}

Outputs nothing. However, if I change the parameter in "struct a" to take a int instead of a string it outputs the int. So why nothing if it's a string?

[keskiverto]

What is the lifetime of the unnamed temporary object that your create on line 16?

Doesn't your compiler say something like:

 In function 'int main()':
16:19: error: taking address of temporary [-fpermissive]

To answer your question:
Nothing is "valid" undefined behaviour.

[volang]

No it runs the program without errors. Is it temporary because I didnt create a copy/instance and used the struct name directly?

[dhayden]

Is it temporary because I didnt create a copy/instance and used the struct name directly?

Correct. &a("hej") says "construct me an a object, call a::a("hej") to initialize it, and take it's address.

[volang]

To bad this way couldn't return a valid copy to use. Less code ;)

[keskiverto]

Make your compiler more verbose and pedantic, if you can.

Unnamed objects live only in the scope of the statement.

You do the equivalent of this:

a* p = nullptr;
{
  a v("hej");
  p = &v;
  cout << p->c; // valid
} // v is destructed here
cout << p->c; // error: dereferencing dangling pointer 

On line 7 the p still has the address that it got on line 4, but the object (v) that was on that memory location does not exists any more.

[Ganado]

Old versions of MSVC and GCC used to only issue a warning diagnostic for taking the address of a temporary, but any even somewhat-recent compiler should error out... I suggest updating your compiler. But even on an outdated compiler, it should at least be a warning, so ALWAYS compile with at least -Wall on GCC, and turn on extra/pedantic too for good measure.

[George P]

@Ganado, I'd add to your suggestions....when using MSVC set the warning level to 4. Anything higher and you get gobs of unnecessary and useless warnings about the C++ standard library.

[keskiverto]

To bad this way couldn't return a valid copy to use. Less code ;)

Drop * and & and you get both less and valid code.

#include <string>
#include <map>
#include <iostream>

struct a {
   std::string c;
};

int main()
{
   std::map<int, a> m;
   m[1] = a{"hej"};
   std::cout << m[1].c;
}

[volang]

keskiverto this was very helpful. Thanks alot