C++ template parameter with default parameters
The problem is that your template template parameter has only two template parameters, as opposed to map
, which has four.
template<class Key, template<class, class, class, class> class Map = std::map>
class MyClass {
};
Or
template<class Key, template<class...> class Map = std::map>
class MyClass {
};
Should compile.
However, to avoid such problems, try to take the map type instead, and extract the key type via the corresponding member typedef. E.g.
template <class Map>
class MyClass {
using key_type = typename Map::key_type;
};
Your code will compile in C++17. A long-standing defect report of the C++ Core Working Group (CWG 150) was resolved (by P0522R0) in time for C++17.
cppreference.com also discuss this here, and include a helpful example:
template<class T> class A { /* ... */ };
template<class T, class U = T> class B { /* ... */ };
template <class ...Types> class C { /* ... */ };
template<template<class> class P> class X { /* ... */ };
X<A> xa; // OK
X<B> xb; // OK in C++17 after CWG 150
// Error earlier: not an exact match
X<C> xc; // OK in C++17 after CWG 150
// Error earlier: not an exact match
Testing with my version of GCC (8.3.0), I find that using the -std=c++17
flag will successfully compile your program; while using earlier versions of C++ (e.g. -std=c++14
or -std=c++11
) will fail.