Say I define a map with a custom comparator such as
struct Obj
{
int id;
std::string data;
std::vector<std::string> moreData;
};
struct Comparator
{
using is_transparent = std::true_type;
bool operator()(Obj const& obj1, Obj const& obj2) { return obj1.id < obj2.id; };
}
std::map<Obj,int,Comparator> compMap;
is there a good way to ensure that downstream users don't have to implement the comparator to use the map as a map?
for instance my compiler throws an error if I try to pass it to a function with a similar type.
template<class T>
inline void add(std::map<T, int>& theMap, T const & keyObj)
{
auto IT = theMap.find(keyObj);
if (IT != theMap.end())
IT->second++;
else
theMap[keyObj] = 1;
}
add(compMap,newObj); //type error here
EDIT:
I kinda over santitized this to make a generic case. and then overlooked the obvious
template<class T, class Comp, class Alloc>
inline void add(std::map<T, int, Comp, Alloc>& theMap, T const & keyObj)
still having issues with one use not being able to deduce T, but went from 80 erros to 1 so... progress
thanks everyone.
You can typedef the specialised type and use that type inplace of
std::map<...
typedef std::map<Obj,int,Comparator> compMap_t;
inline void add(compMap_t& theMap, Obj const & keyObj)
...
Downstream users either use the type declared by you
using my_important_map = std::map<Obj,int,Comparator>;
or better use functions which take a generic map type,
auto some_function(auto const& map_)
{
//do something with the map and don't care about the ordering
return map_.find(Obj(1));
}
My situation:
I frequently need to have a vector of structures where one field can be thought of as a Key or ID, and rather than store it expensively in a map (memory usage is very important in this app) I want to store it in a flat vector but present a map-like interface for finding elements by key.
My first solution to this problem:
template <class T, class Key, class KeyFn>
class TKeyedVector : public std::vector<T>
{
public:
const_iterator find(const Key& key) const {return std::find_if(begin(), end(), [&](const T& entry) {return keyFn(entry)==key; }); }
KeyFn keyFn;
};
struct KeyedDataEntry
{
std::string key;
int value;
struct KeyExtractor {
const std::string& operator()(const KeyedDataEntry& e) const {return e.key; };
};
};
using KeyedDataArray = TKeyedVector<KeyedDataEntry, std::string, KeyedDataEntry::KeyExtractor>;
Now this all works, but I would like to be able to remove the need for the KeyExtractor type by using the pointer to the member variable embedded into the type:
template <class T, class Key, Key T::* keyFn>
class TKeyedVector : public std::vector<T>
{
public:
const_iterator find(const Key& key) const {return std::find_if(begin(), end(), [&](const T& entry) {return keyFn(entry)==key; }); }
};
using KeyedDataArray = TKeyedVector<KeyedDataEntry, std::string, &KeyedDataEntry::key>;
However I can't get this to work. I've been looking at the implementation of std::mem_fn for clues, but I can't work out how to do it. The error I get with is something like:
warning C4353: nonstandard extension used: constant 0 as function expression. Use '__noop' function intrinsic instead
Any clues?
EDIT: sample version at http://ideone.com/Qu6TEy
Here is the start of a working solution. You don't need a special extractor object.
Note that I have encapsulated the vector. In time, you'll regret not doing this.
#include <vector>
#include <string>
template <class T, class Key, const Key& (T::*Extractor)() const>
class TKeyedVector
{
using storage = std::vector<T>;
using const_iterator = typename storage::const_iterator;
public:
decltype(auto) begin() const
{
return storage_.begin();
}
decltype(auto) end() const
{
return storage_.end();
}
const_iterator find(const Key& key) const
{
return std::find_if(begin(),
end(),
[&](const T& entry)
{
return entry.*Extractor() == key;
});
}
storage storage_;
};
struct KeyedDataEntry
{
std::string key;
int value;
const std::string& get_key() const { return key; }
};
int main()
{
TKeyedVector<KeyedDataEntry, std::string, &KeyedDataEntry::get_key> mymap;
}
But there is a problem with this idea of yours.
In order for this structure to be a map, the keys must be immutable. This argues for only returning immutable objects. This then argues immediately for simply using an unordered_set or set.
If you're going to return references to mutable objects in the underlying vector, then you may as well simply use std::find_if with a predicate to find them.
A pointer to member requires the pointer to member call syntax. (entry.*keyFn)()
C++17 will come with a standard std::invoke function to make writing such templates a bit less tiresome (it will work for all callable objects). But in the meanwhile, this is how you need to do this.
Is there a way to specify the default value std::map's operator[] returns when an key does not exist?
While this does not exactly answer the question, I have circumvented the problem with code like this:
struct IntDefaultedToMinusOne
{
int i = -1;
};
std::map<std::string, IntDefaultedToMinusOne > mymap;
No, there isn't. The simplest solution is to write your own free template function to do this. Something like:
#include <string>
#include <map>
using namespace std;
template <typename K, typename V>
V GetWithDef(const std::map <K,V> & m, const K & key, const V & defval ) {
typename std::map<K,V>::const_iterator it = m.find( key );
if ( it == m.end() ) {
return defval;
}
else {
return it->second;
}
}
int main() {
map <string,int> x;
...
int i = GetWithDef( x, string("foo"), 42 );
}
C++11 Update
Purpose: Account for generic associative containers, as well as optional comparator and allocator parameters.
template <template<class,class,class...> class C, typename K, typename V, typename... Args>
V GetWithDef(const C<K,V,Args...>& m, K const& key, const V & defval)
{
typename C<K,V,Args...>::const_iterator it = m.find( key );
if (it == m.end())
return defval;
return it->second;
}
C++17 provides try_emplace which does exactly this. It takes a key and an argument list for the value constructor and returns a pair: an iterator and a bool.: http://en.cppreference.com/w/cpp/container/map/try_emplace
The C++ standard (23.3.1.2) specifies that the newly inserted value is default constructed, so map itself doesn't provide a way of doing it. Your choices are:
Give the value type a default constructor that initialises it to the value you want, or
Wrap the map in your own class that provides a default value and implements operator[] to insert that default.
The value is initialized using the default constructor, as the other answers say. However, it is useful to add that in case of simple types (integral types such as int, float, pointer or POD (plan old data) types), the values are zero-initialized (or zeroed by value-initialization (which is effectively the same thing), depending on which version of C++ is used).
Anyway, the bottomline is, that maps with simple types will zero-initialize the new items automatically. So in some cases, there is no need to worry about explicitly specifying the default initial value.
std::map<int, char*> map;
typedef char *P;
char *p = map[123],
*p1 = P(); // map uses the same construct inside, causes zero-initialization
assert(!p && !p1); // both will be 0
See Do the parentheses after the type name make a difference with new? for more details on the matter.
There is no way to specify the default value - it is always value constructed by the default (zero parameter constructor).
In fact operator[] probably does more than you expect as if a value does not exist for the given key in the map it will insert a new one with the value from the default constructor.
template<typename T, T X>
struct Default {
Default () : val(T(X)) {}
Default (T const & val) : val(val) {}
operator T & () { return val; }
operator T const & () const { return val; }
T val;
};
<...>
std::map<KeyType, Default<ValueType, DefaultValue> > mapping;
More General Version, Support C++98/03 and More Containers
Works with generic associative containers, the only template parameter is the container type itself.
Supported containers: std::map, std::multimap, std::unordered_map, std::unordered_multimap, wxHashMap, QMap, QMultiMap, QHash, QMultiHash, etc.
template<typename MAP>
const typename MAP::mapped_type& get_with_default(const MAP& m,
const typename MAP::key_type& key,
const typename MAP::mapped_type& defval)
{
typename MAP::const_iterator it = m.find(key);
if (it == m.end())
return defval;
return it->second;
}
Usage:
std::map<int, std::string> t;
t[1] = "one";
string s = get_with_default(t, 2, "unknown");
Here is a similar implementation by using a wrapper class, which is more similar to the method get() of dict type in Python: https://github.com/hltj/wxMEdit/blob/master/src/xm/xm_utils.hpp
template<typename MAP>
struct map_wrapper
{
typedef typename MAP::key_type K;
typedef typename MAP::mapped_type V;
typedef typename MAP::const_iterator CIT;
map_wrapper(const MAP& m) :m_map(m) {}
const V& get(const K& key, const V& default_val) const
{
CIT it = m_map.find(key);
if (it == m_map.end())
return default_val;
return it->second;
}
private:
const MAP& m_map;
};
template<typename MAP>
map_wrapper<MAP> wrap_map(const MAP& m)
{
return map_wrapper<MAP>(m);
}
Usage:
std::map<int, std::string> t;
t[1] = "one";
string s = wrap_map(t).get(2, "unknown");
One workaround is to use map::at() instead of [].
If a key does not exist, at throws an exception.
Even nicer, this also works for vectors, and is thus suited for generic programming where you may swap the map with a vector.
Using a custom value for unregistered key may be dangerous since that custom value (like -1) may be processed further down in the code. With exceptions, it's easier to spot bugs.
Expanding on the answer https://stackoverflow.com/a/2333816/272642, this template function uses std::map's key_type and mapped_type typedefs to deduce the type of key and def.
This doesn't work with containers without these typedefs.
template <typename C>
typename C::mapped_type getWithDefault(const C& m, const typename C::key_type& key, const typename C::mapped_type& def) {
typename C::const_iterator it = m.find(key);
if (it == m.end())
return def;
return it->second;
}
This allows you to use
std::map<std::string, int*> m;
int* v = getWithDefault(m, "a", NULL);
without needing to cast the arguments like std::string("a"), (int*) NULL.
Pre-C++17, use std::map::insert(), for newer versions use try_emplace(). It may be counter-intuitive, but these functions effectively have the behaviour of operator[] with custom default values.
Realizing that I'm quite late to this party, but if you're interested in the behaviour of operator[] with custom defaults (that is: find the element with the given key, if it isn't present insert a chosen default value and return a reference to either the newly inserted value or the existing value), there is already a function available to you pre C++17: std::map::insert(). insert will not actually insert if the key already exists, but instead return an iterator to the existing value.
Say, you wanted a map of string-to-int and insert a default value of 42 if the key wasn't present yet:
std::map<std::string, int> answers;
int count_answers( const std::string &question)
{
auto &value = answers.insert( {question, 42}).first->second;
return value++;
}
int main() {
std::cout << count_answers( "Life, the universe and everything") << '\n';
std::cout << count_answers( "Life, the universe and everything") << '\n';
std::cout << count_answers( "Life, the universe and everything") << '\n';
return 0;
}
which should output 42, 43 and 44.
If the cost of constructing the map value is high (if either copying/moving the key or the value type is expensive), this comes at a significant performance penalty, which would be circumvented with C++17's try_emplace().
If you have access to C++17, my solution is as follows:
std::map<std::string, std::optional<int>> myNullables;
std::cout << myNullables["empty-key"].value_or(-1) << std::endl;
This allows you to specify a 'default value' at each use of the map. This may not necessarily be what you want or need, but I'll post it here for the sake of completeness. This solution lends itself well to a functional paradigm, as maps (and dictionaries) are often used with such a style anyway:
Map<String, int> myNullables;
print(myNullables["empty-key"] ?? -1);
Maybe you can give a custom allocator who allocate with a default value you want.
template < class Key, class T, class Compare = less<Key>,
class Allocator = allocator<pair<const Key,T> > > class map;
With C++20 it is simple to write such getter:
constexpr auto &getOrDefault(const auto &map, const auto &key, const auto &defaultValue)
{
const auto itr = map.find(key);
return itr == map.cend() ? defaultValue : itr->second;
}
Here is a correct approach that will conditionally return a reference if the caller passes in an lvalue reference to the mapped type.
template <typename Map, typename DefVal>
using get_default_return_t = std::conditional_t<std::is_same_v<std::decay_t<DefVal>,
typename Map::mapped_type> && std::is_lvalue_reference_v<DefVal>,
const typename Map::mapped_type&, typename Map::mapped_type>;
template <typename Map, typename Key, typename DefVal>
get_default_return_t<Map, DefVal> get_default(const Map& map, const Key& key, DefVal&& defval)
{
auto i = map.find(key);
return i != map.end() ? i->second : defval;
}
int main()
{
std::map<std::string, std::string> map;
const char cstr[] = "world";
std::string str = "world";
auto& ref = get_default(map, "hello", str);
auto& ref2 = get_default(map, "hello", std::string{"world"}); // fails to compile
auto& ref3 = get_default(map, "hello", cstr); // fails to compile
return 0;
}
If you would like to keep using operator[] just like when you don't have to specify a default value other than what comes out from T() (where T is the value type), you can inherit T and specify a different default value in the constructor:
#include <iostream>
#include <map>
#include <string>
int main() {
class string_with_my_default : public std::string {
public:
string_with_my_default() : std::string("my default") {}
};
std::map<std::string, string_with_my_default> m;
std::cout << m["first-key"] << std::endl;
}
However, if T is a primitive type, try this:
#include <iostream>
#include <map>
#include <string>
template <int default_val>
class int_with_my_default {
private:
int val = default_val;
public:
operator int &() { return val; }
int* operator &() { return &val; }
};
int main() {
std::map<std::string, int_with_my_default<1> > m;
std::cout << m["first-key"] << std::endl;
++ m["second-key"];
std::cout << m["second-key"] << std::endl;
}
See also C++ Class wrapper around fundamental types