std :: map默认值 [英] std::map default value

问题描述

有没有办法指定默认值 std :: map 的 operator [] 键不存在？

Is there a way to specify the default value std::map's operator[] returns when an key does not exist?

推荐答案

不，没有。最简单的解决方案是编写自己的自由模板函数来做到这一点。像：

No, there isn't. The simplest solution is to write your own free template function to do this. Something like:

#include <string>
#include <map>
using namespace std;

template <typename K, typename V>
V GetWithDef(const  std::map <K,V> & m, const K & key, const V & defval ) {
   typename std::map<K,V>::const_iterator it = m.find( key );
   if ( it == m.end() ) {
      return defval;
   }
   else {
      return it->second;
   }
}

int main() {
   map <string,int> x;
   ...
   int i = GetWithDef( x, string("foo"), 42 );
}

---

+11更新

用途：用于通用关联容器以及可选的比较器和分配器参数。

Purpose: Account for generic associative containers, as well as optional comparator and allocator parameters.

template <template<class,class,class...> class C, typename K, typename V, typename... Args>
V GetWithDef(const C<K,V,Args...>& m, K const& key, const V & defval)
{
    typename C<K,V,Args...>::const_iterator it = m.find( key );
    if (it == m.end())
        return defval;
    return it->second;
}

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