为模板化函数使用默认参数

Question

I have two functions which look very similar to me :

template <typename Predicate>
vector<int> FindTopItems(const string& query,
                         const Predicate& predicate) const;
                         
vector<int> FindTopItems(const string& query,
                         ItemStatus status = ItemStatus::Initial) const;

It is possible to use only templated version of this function? How to set it's predicate to default value, set by enum class?

So I want the following code to be compilable :

int main() {

   FindTopItems(""s, [](){}); /* predicate version */
   FindTopItems(""s); /* default parameter as enum class should be used */
   
   return 0;
}

Answer 1

You can set the default value for predicate to ItemStatus , then make a helper class that returns your default value.

#include <iostream>
#include <vector>
#include <string>

enum class ItemStatus {
    Initial
};

template <typename T>
auto defaultValue() {
    return T{};
}

template <>
auto defaultValue<ItemStatus>() {
    return ItemStatus::Initial;
}

template <typename Predicate = ItemStatus>
std::vector<int> FindTopItems(const std::string& query, const Predicate& predicate = defaultValue<Predicate>()) {
    if constexpr (std::is_same_v<Predicate, ItemStatus>) {
        std::cout << "ItemStatus " << static_cast<int>(predicate) << '\n';
    } else {
        std::cout << "Predicate " << predicate() << '\n';
    }
    return {};
}

int main() {
   FindTopItems(std::string{}, [](){ return 1; }); /* predicate version */
   FindTopItems(std::string{}); /* default parameter as enum class should be used */
   
   return 0;
}