c++11 I can not initialize a member with '()' while i can use '{}' to do this? [duplicate]

#pragma once
#include <list>
#include <unordered_map>
template<typename K, typename V>
class LRUCache
{
public:
    LRUCache(size_t _capcity):capcity(_capcity)
    {   
    }
    ~LRUCache()
    {
    
    }
    void put(K key ,V value)
    {
        if (dic.count(key))
        {
            auto it = dic[key];
            *it = { key, value };
            data.splice(data.begin(), data, it);
        }
        else
        {
            data.push_front({ key, value });
            dic[key] = data.begin();
        }
        while (data.size() > capcity)
        {
            dic.erase(data.back().first);
            data.pop_back();
        }
    }

    V get(K key)
    {
        if (!dic.count(key)) return -1;
        auto it = dic[key];
        data.splice(data.begin(), data, it);
        return it->second;
    }


private:
    size_t  capcity;
    std::list<std::pair<K, V>> data;
    std::unordered_map<K, typename std::list<std::pair<K, V>>::iterator> dic;
};

This class has a constructor with initial list.

#pragma once
#include "LRUCache.h"

class SLRU
{
public:
    SLRU();
    ~SLRU();

private:
    LRUCache<std::string, int> protectLRU{ 10 };
    LRUCache<std::string, int> eliminateLRU{ 10 };

};

SLRU::SLRU()
{
}

SLRU::~SLRU()
{
}

this class wants to initialize a LRUCache member.

i find that if the LRUCache's constructor has no initial list

    LRUCache(size_t _capcity)
    {   
           capcity = _capcity;
    }

i can use '()' to initialize the LRUCache member

LRUCache<std::string, int> eliminateLRU(10);

otherwise, i can not use '()'. so what magic does '{}‘ has betwen it and initial list? where can i find the answer when meeting the problem like it?