通常，如果你做一个查找和插入，那么你想保留（和检索）旧的值，如果它已经存在。如果你只是想覆盖任何旧的值， map [foo_obj] =一些值会这样做。

Normally if you do a find and maybe an insert, then you want to keep (and retrieve) the old value if it already existed. If you just want to overwrite any old value, map[foo_obj]="some value" will do that.

typedef std::map<Foo*,std::string> M;
typedef M::iterator I;
std::pair<I,bool> const& r=my_map.insert(M::value_type(foo_obj,"some value"));
if (r.second) { 
    // value was inserted; now my_map[foo_obj]="some value"
} else {
    // value wasn't inserted because my_map[foo_obj] already existed.
    // note: the old value is available through r.first->second
    // and may not be "some value"
}
// in any case, r.first->second holds the current value of my_map[foo_obj]

您可能想要使用辅助函数：

This is a common enough idiom that you may want to use a helper function:

template <class M,class Key>
typename M::mapped_type &
get_else_update(M &m,Key const& k,typename M::mapped_type const& v) {
    return m.insert(typename M::value_type(k,v)).first->second;
}

get_else_update(my_map,foo_obj,"some value");

如果你有一个昂贵的计算v你想跳过，如果它已经存在你可以概括一下：

If you have an expensive computation for v you want to skip if it already exists (e.g. memoization), you can generalize that too:

template <class M,class Key,class F>
typename M::mapped_type &
get_else_compute(M &m,Key const& k,F f) {
   typedef typename M::mapped_type V;
   std::pair<typename M::iterator,bool> r=m.insert(typename M::value_type(k,V()));
   V &v=r.first->second;
   if (r.second)
      f(v);
   return v;
}

其中eg

struct F {
  void operator()(std::string &val) const 
  { val=std::string("some value")+" that is expensive to compute"; }
};
get_else_compute(my_map,foo_obj,F());

如果映射类型不是默认可构造的，那么make F提供一个默认值，参数get_else_compute。

If the mapped type isn't default constructible, then make F provide a default value, or add another argument to get_else_compute.