How to let map container create a new object using different constructor?

Question

When we try to access an object in a map container using a key while that object does not exist, it is automatically constructed using default constructor.

I wonder is there a way to use another constructor, for example, with one parameter?

The trivial solution is to wrap the value type, and let `Derived::Derived()` call `Base::Base(param)`. — MSalters, Mar 29 '13 at 10:26
std::map _value-initializes_ object values using default constructor. So not possible. For references to C++ standards please also see - [link](http://stackoverflow.com/questions/9025792/does-the-default-constructor-of-stdpair-set-basic-types-int-etc-to-zero) — Amar, Mar 30 '13 at 03:30

score 2 · Accepted Answer · edited Mar 29 '13 at 08:54

2

I personally think this behavior is weird, but if you really want to do so, I suggest that explicitly write what you want:

For example:

typedef std::map<int, std::string> MapType;

std::string FindSomething(int key, const std::string& extraParameter)
{
    MapType::iterator It = TheMap.find(key);

    if (It == TheMap.end())
    {
        TheMap.insert(std::make_pair(key, extraparameter));

        return extraParameter;
    }
    else
    {
        return It->second;
    }
}

Do not depend on the automatically insertion.

edited Mar 29 '13 at 08:54

W.B.

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answered Mar 29 '13 at 07:58

Marson Mao

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You could also make FindSomething a template function. – W.B. Mar 29 '13 at 08:53

How to let map container create a new object using different constructor?

1 Answers1