Create a map of objects without destroingthe objects

Question

I am trying to create a map which contains the objects with different arguments.

But I found that after inserting the pair, the object will be destroyed.

If I try to use the function in this object.For example:

#include <map>
#include <iostream>

class Test{
    public:
        Test(double value) : value_(value){}
        ~Test(){std::cout<< "destroyed";}
        void plusone()  {value_ += 1;}
    private:
        double value_;
};

int main(){
    std::map<long, Test> map;

    map.insert(std::make_pair(1, Test(1.2)));
    map[1].plusone();
    return 0;
}

It will show: [Error] no matching function for call to 'Class::Class()'

[Note] candidate expects 1 argument, 0 provided

How can I do this?

In your case, it seems that you may use `map::at` (throw if not found) instead of `map::operator[]` (try to insert default object, and return inserted/found object). — Jarod42, Mar 02 '16 at 00:32

score 1 · Answer 1 · answered Mar 01 '16 at 23:52

1

The syntax map[1] can only be used when the mapped type has a default constructor. This is because if the key is not found then a default-constructed object is inserted, but the compiler has to generate code for this at compile-time.

If you do not want to add a default constructor then you have to use different lookup code, e.g.:

auto it = map.find(1);
if ( it != map.end() )
    it->second.memberfuncion();

Also, your error message has nothing to do with destroying objects, as your title mentions.

In the code map.insert(std::make_pair(1, Class(argument)));, you create a temporary object Class(argument), and copy that into the map. Then the temporary object is destroyed.

If you want to avoid this and construct directly in the map, use map.emplace(1, argument);.

answered Mar 01 '16 at 23:52

M.M

138,810
21
208
365

Note that `emplace()` is only available in C++11 and later. – Remy Lebeau Mar 02 '16 at 00:08
thank you so much ,very detail explanation. btw which one you prefer ? syntax [ ] or .find – CHOCOLEO Mar 02 '16 at 00:14
yes, in C++03 there is no option but to copy the object in – M.M Mar 02 '16 at 00:14
@CHOCOLEO well I prefer `[]` personally but as I explain in my answer, for some `Class`es it is not possible – M.M Mar 02 '16 at 00:15
@M.M Would you like to give one example? as I though there is no harm adding a default constructor to the class. Thanks – CHOCOLEO Mar 02 '16 at 00:20
@CHOCOLEO my answer includes an example of `find`, and your question includes an example of `[]` (if you added a default constructor) – M.M Mar 02 '16 at 00:46

score 0 · Answer 2 · answered Mar 01 '16 at 23:57

0

Item access via map.operator[] requires your mapped type to be default constructible.

If it is not default constructible, use map.find which has no such requirement.

answered Mar 01 '16 at 23:57

donkopotamus

22,114
2
48
60

Create a map of objects without destroingthe objects

2 Answers2