std::map is a class in the C++ Standard Library. It is a sorted associative container that contains key-value pairs with unique keys. Search, removal, and insertion operations have logarithmic complexity. Maps are usually implemented as red-black trees.
Questions tagged [stdmap]
1446 questions
72
votes
6 answers
Find mapped value of map
Is there a way in C++ to search for the mapped value (instead of the key) of a map, and then return the key? Usually, I do someMap.find(someKey)->second to get the value, but here I want to do the opposite and obtain the key (the values and keys are…
wrongusername
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votes
3 answers
Checking for existence in std::map - count vs find
So there seem to be two generally acceptable methods of determining whether or not a key exists in a std::map:
map.find(key) != map.end()
map.count(key) > 0
Is one more efficient than the other? Specifically, the concept of count() could be…
dolphy
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67
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3 answers
How can I delete elements of a std::map with an iterator?
I would like to loop through an std::map and delete items based on their contents. How best would this be done?
user542687
63
votes
11 answers
Use of for_each on map elements
I have a map where I'd like to perform a call on every data type object member function. I yet know how to do this on any sequence but, is it possible to do it on an associative container?
The closest answer I could find was this: Boost.Bind to…
Antonio Pérez
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62
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8 answers
When I should use std::map::at to retrieve map element
I have read different articles on web and questions at stackoverflow, but for me it is not clear is there any exclusive case when it is better to use std::map::at to retrieve map element.
According to definition, std::map::at
Returns a reference…
T M
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62
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8 answers
How to iterate over a std::map full of strings in C++
I have the following issue related to iterating over an associative array of strings defined using std::map.
-- snip --
class something
{
//...
private:
std::map table;
//...
}
In the constructor I populate table…
crazybyte
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59
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6 answers
Why Can't I store references in a `std::map` in C++?
I understand that references are not pointers, but an alias to an object. However, I still don't understand what exactly this means to me as a programmer, i.e. what are references under the hood?
I think the best way to understand this would be to…
ng5000
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59
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10 answers
What is the best way to use two keys with a std::map?
I have a std::map that I'm using to store values for x and y coordinates. My data is very sparse, so I don't want to use arrays or vectors, which would result in a massive waste of memory. My data ranges from -250000 to 250000, but I'll only have a…
Roland Rabien
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53
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7 answers
What is the difference between std::list and std::map in C++ STL?
What is the difference between std::list and std::map? Is there a find method for the list, too?
Boolean
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C++ std::map holding ANY type of value
Basically I want MyClass that holds a Hashmap that maps Field name(string) to ANY type of
Value.. For this purpose I wrote a separate MyField class that holds the type & value information..
This is what I have so far:
template
class…
user3794186
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50
votes
6 answers
C++ map vs map performance (I know, "again?")
I was using a map with a std::string key and while everything was working fine I wasn't getting the performance I expected. I searched for places to optimize and improved things only a little and that's when a colleague said, "that string key is…
uroc
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46
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7 answers
How can i estimate memory usage of std::map?
For example, I have a std::map with known sizeof(A) and sizeof(B), while map has N entries inside. How would you estimate its memory usage?
I'd say it's something like
(sizeof(A) + sizeof(B)) * N * factor
But what is the factor? Different formula…
Drakosha
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45
votes
2 answers
std::map thread-safety
Is reference to object in std::map is thread safe?
std::map< std::string, Object > _objects;
map can be changed from many threads and this access is synchronized, but reference to value (Object &) accessable just from 1 instance and thread. is…
dr11
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44
votes
6 answers
How can I traverse/iterate an STL map?
I want to traverse an STL map. I don't want to use its key. I don't care about the ordering, I just look for a way to access all elements it contains. How can I do this?
atoMerz
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44
votes
5 answers
advantages of std::set vs vectors or maps
This may be a stupid question, I am quite new to C++ and programming in general.
I wish to understand the use of several STL containers and with that in mind, I was wondering what the advantages are of using std::set vs for example using vectors or…
brunodd
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