I would like to make my app's std::map
s' keys not be int
s, but rather be more strongly typed as templated non-type enum
s defined as a member of a struct
. The first program below shows the concept of how my app currently uses map
s. It compiles and runs ok.
#include <map>
template< int >
struct NummedMap
{
typedef std::map< int, NummedMap > NumableMap;
NummedMap() {}
NumableMap numableMap;
};
int main()
{
NummedMap< 3 > numableMap3;
NummedMap< 4 > numableMap4;
numableMap3.numableMap[ 3 ] = numableMap3;
numableMap4.numableMap[ 4 ] = numableMap4;
return 0;
}
The second program shows how I would like to program my app's map
s, but I am missing some concepts regarding non-type templates and why an < enum EnumT >
is different from a POD int
.
#include <map>
struct Enums1 // a struct containing one kind scoped enums
{
enum class Action
{
AAAA,
BBBB
};
};
struct Enums2 // a struct containing another kind scoped enums
{
enum class Action
{
CCCC,
DDDD
};
};
template< enum EnumT >
struct EnummedMap // a struct containing a map whose key is non-type templateable
{
typedef std::map< EnumT, EnummedMap > EnumableMap; // error C2065: 'EnumT': undeclared identifier
EnummedMap() {}
EnumableMap enumableMap;
};
int main()
{
EnummedMap< Enums1::Action > enummedMap1; // error C2993: illegal type for non-type template parameter
EnummedMap< Enums2::Action > enummedMap2; // error C2993: illegal type for non-type template parameter
enummedMap1.enumableMap[ Enums1::Action::AAAA ] = enummedMap1; // error C2678: binary '[': no operator found which takes a left-hand operand of type
enummedMap2.enumableMap[ Enums2::Action::CCCC ] = enummedMap2; // error C2678: binary '[': no operator found which takes a left-hand operand of type
return 0;
}
I don't understand why EnumableMap
's key is undeclared, or why for example Enums1::Action
does not function roughly like an int
key.
template< enum EnumT >
struct EnummedMap // a struct containing a map whose key is non-type templateable
{
typedef std::map< EnumT, EnummedMap > EnumableMap;
A non-type template parameter (in this case an old-style enum), is a single value, and by definition is not a type, but std::map expects the Key to be a type, not a value. To make this work, change "enum" to "typename":
template<typename EnumT > // << *** HERE ***
struct EnummedMap // a struct containing a map whose key is non-type templateable
{
typedef std::map< EnumT, EnummedMap > EnumableMap;
EnummedMap() {}
EnumableMap enumableMap;
};
However, this allows for non-enum types. If you want to prevent all uses EXCEPT for enum types, you can use a static_assert:
#include <type_traits>
//...
template<typename EnumT>
struct EnummedMap
{
static_assert(std::is_enum_v<EnumT>); // c++17
//static_assert(std::is_enum<EnumT>::value, ""); // c++11
typedef std::map< EnumT, EnummedMap > EnumableMap;
EnummedMap() {}
EnumableMap enumableMap;
};
Then it won't compile if a non-enum is passed as the template parameter.
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