Re: Initialising map member without copy
* Paul Brettschneider:
Alf P. Steinbach wrote:
* Paul Brettschneider:
Hello,
I have a use-case where I have a number of objects of a complex class.
Copying is very expensive and should be avoided. I want to access those
objects by ids (std::string objects). A std::map<std::string, MyClass>
object suggests itself but, at least under gcc, it does two gratuitous
copies:
#include <iostream>
#include <map>
#include <string>
class Test {
int x;
public: Test() : x(0) { std::cout << "Default!\n"; };
Test(const Test &t) : x(t.x) { std::cout << "Copy!\n"; };
void test() { std::cout << x << "\n"; }
};
int main()
{
std::map<std::string, Test> test_map;
test_map["hello"].test();
return 0;
}
outputs:
Default!
Copy!
Copy!
0
I suppose the first copy happens at the initialisation of a
std::pair<key, value> the second one when the pair is moved into the
container proper.
For me it's not obvious why the object couldn't just be created in-place.
Does the standard say anything about this, or is this an QOI issue?
And what is the common way of avoiding this kind of problem?
I've pondered about either using "smart" pointers (i.e. refcounting), or
making the copy constructor aware of the fact that it is copying a
freshly created instance, and use init() function on the object once it
is in place. Both methods seem unnecessarily complicated. :(
Store smart-pointers, e.g. boost::shared_ptr.
Up to now I tried to avoid the dependency on external libraries like boost
as far as possible. But it does seem to have the class that is needed for
this case: boost::ptr_map, so maybe it's time to bite the bullet...
Ah, didn't know about ptr_map, but then, never needed it.
Amazing what exists!
:-)
Cheers,
- Alf
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