How do I specify a default value for a template template parameter?

I have a function which takes a template template parameter, e.g.:

template <template <typename K, typename V> typename Map>
void foo();

I want to specify a default value for Map; namely, std::unordered_map<K,V>, which indeed has two template parameters itself. What's the syntax for that?

The syntax is for you to use just the name of the template you want, without any angle brackets.

So, for your example, you would write:

#include <unordered_map>

template <template <typename K, typename V> typename Map = std::unordered_map>
void foo();

and this should compile. Full example program:

#include <unordered_map>
#include <iostream>

template <template <typename K, typename V> typename Map = std::unordered_map>
int foo() { 
    return sizeof(Map<int, double>);
}

int main() {
    std::cout << "size of (std::unordered_map<int, double> is: "
        << sizeof(std::unordered_map<int,double>) << '\n';
    std::cout << "size of default Map<int, double> is: " << foo() << '\n';
}

GodBolt.