stl map questions.

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The currently used as follows.

C++

map<string, Material*> m_mapMaterial;
...
Material* D3D::GetMaterialInMap(string fileName)
{
     if (m_mapMaterial[fileName]) 
          return m_mapMaterial[fileName];
     return NULL;
}

Takes the data using the key.
Conversely, there can be obtained a key to use the data? (for the map<string,> m_mapMaterial;)
Or do I need to do the following?

C++

map<string, pair<string, Material*>> m_mapMaterial; // string of map == string of pair

Let me know if you have any better way!

Posted 7-Sep-14 1:02am

무명

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CPallini 7-Sep-14 7:12am

What do you mean with: "Conversely, there can be obtained a key to use the data?" ? The map provides iterators to access its items (both key and value).

무명 7-Sep-14 7:50am

- No key value.
- Data values exist.
In this situation, seek the key value in the map.

CPallini 7-Sep-14 7:57am

In a map< string key material * pvalue > if you need the key corresponding to pvalue then you might either:
- iterate over all the keys in order to find the item having the given pvalue
or
- use another map, namely map< Material, string> to perform the inverse look-up.

무명 7-Sep-14 8:37am

As compared with the second method, the following methods are not good?
map < string, pair < string, Material* > >

[use another map, namely map <material, string=""> to perform the inverse look-up.]
If I use the above method ...

map < string, Material* > a;
map < Material*, string > b;
...
string GetString(Material* pMaterial)
{
return b[pMaterial];
}
void SetMap(Material* pMaterial, string str)
{
set a, b;
}

It seems confusing.

Sergey Alexandrovich Kryukov 7-Sep-14 11:20am

Please, why would you need to obtain a key from data? You could if you stored key in data, but why storing double data? I don't see where would you really need it.
—SA

무명 7-Sep-14 20:28pm

map < string, Material* > m_mapMaterial;
Until now , there was no work required string using the above code .
It is necessary now.
I need to output a string to MFC Edit Control.
The problem is that it 's rarely needed.

map < string, Material* > m_mapMaterial.
Therefore , were questions about whether you can be solved by using only the above code.
The first method CPallini told because of performance issues , can not be used .

KarstenK 7-Sep-14 15:44pm

I would use a second map:

map< Material*,string> m_mapNames;

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Sergey Alexandrovich Kryukov · Accepted Answer · 2014-09-07T15:08:00

In some cases, you really need to find not only the value by the key, but key by value. But it simply means that… the value should play the role of the key. First of all, it's only possible if the values are also unique, otherwise it is unknown which of the two or more map elements should be chosen by some value.

To achieve such functionality, you need to define a "two-way, one-to-one map", which can be easily implemented by composing two maps, for example: map <string, Material*> and map <Material*, string>. Naturally, each element composed of the string and Material* pair should be added in two maps at the same time, and uniqueness should be checked up. Also, you should taken about identity of Material part: if, by some reason, you try to add two different pointers to the same Material object, they will behave as two different object, from the standpoint of map functionality.

—SA