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# Help to understand my own misstake?

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Hi,

Today/Yesterday I been having a few problem in my code which I manged to "solve", but I was wondering why my original approach didnt work?

Basically I have this std::map with a vector inside it like

std::map<int, std::vector<GameObject*>> testMap;


than i started to insert stuffs like so

mapTest.insert(std::pair<int, std::vector<GameObject*>>(renderID,{gameObjectTest})); //first try
mapTest.insert(mapTest.end(),std::pair<int, std::vector<GameObject*>>(renderID,{gameObjectTest})); //second try


I also tried to have a tempVector where I push_back the gameObjectTest in it and just had tempVector instead of { gameObjectTest }

At this point I got no syntax error or the likes, and when I ran the code it works great.

But when I tried to access the gameobject via

mapTest.at(0).at(0);


I get a std::out_of_range error,

but instead of using .at I tried to use

std::map<int, std::vector<GameObject*>>::iterator it = mapTest.begin();
it->second.at(0);


for now it works and I get no std::out_of_range error. I was wondering how come .at doesnt work but .begin (aka iterator) works?

Edited by Conny14156

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http://en.cppreference.com/w/cpp/container/map/at

std::map::at takes a key, not an index. What your first try was actually accomplishing was returning the value associated with key=0. In the words of that reference:

std::out_of_range if the container does not have an element with the specified key

You didn't have key=0 in your std::map hence that exception. std::map::begin, however, returns an iterator to the first std::pair in the std::map container. The key isn't used at all. As long as there is one item in your std::map the iterator method will work.

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i feel stupid and thanks >.<

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Unless you can guarantee that the particular key you want is in the map you may instead want to consider using find and checking the returned iterator is valid.

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