Question

What is the best way in C++ to use std::map in loops ?

• dynamically allocated
• stack allocated

Code:

for(int i=0;i<3;i++)
{
  std::map<int,int>* m = new std::map<int,int>;
  //or ...
  std::map<int,int> m;

}

Answer 1

Avoid new unless you really need it, i.e. the variable/structure has a lifetime unrelated to any calling scope. (If it “belongs” to the calling function, return by value.)

This is clearly not such a case. The second, preferable, example is called a local variable.

I would be making a choice between

for(int i=0;i<3;i++)
{
  std::map<int,int> m;
  …
}

and

std::map<int,int> m;
for(int i=0;i<3;i++)
{
  …
  m.clear();
}

The latter may perform better when the container is std::vector by reusing allocated memory. With map the difference is only style.