为什么std :: map有一个find成员函数？

Question

A colleague and I were discussing the relative merits of member vs. non-member functions. A question arose: why does std::map have a find member function.

My answer was that although you can use std::find on maps, you must search for the key-value pair, or use find_if and eg a lambda. However, this is linear and map.find offers a search by key in better than linear time. I ended with the assertion that if it could have been a non-member, then it would have been! (Although, std::string suggests that I might have been somewhat hasty in my generalization).

My colleague pointed out that it would be possible to implement find the same way as a non-member function using map.lower_bound .

Is there a rationale for map.find having been made a member?

Answer 1

A big objection to implementing std::find searching for a key on std::map as a non-member function is that doing so would prevent you from implementing the current version of std::find , which searches for a key-value pair.

Being an associative container, std::map contains key-value pairs. Non-member std::find is defined for all containers as a function that searches for an item in the container, which must be a key-value pair for std::map ; using std::find to look up an item by its key would be inconsistent.

Obviously, one could implement std::find_by_key function applicable only to maps, but such function would always have a specialization based on the type of map. This provides no improvement in the API design over adding a member-function.

Answer 2

My colleague pointed out that it would be possible to implement find the same way as a non-member function using map.lower_bound .

This would lead to inconsistency across different types of maps. For instance, you cannot implement a free function find using the lower_bound member function for std::unordered_map , since there is no such member function (same for google::dense_hash_map etc.). Efficient find for unordered_map needs access to class internals and needs to know its implementation details. Then, you would end up with map not having find member function and unordered_map having one. This would make hard to write a generic code that would allow user to choose between different types of maps, such as:

template <class map_type>
void (const map_type & map) {
   // use std::find or map.find here ???

Answer 3

std::map has a member find because map::operator[] and map::at don't cover all the use cases for looking up by key.

operator[] requires the mapped_type be DefaultConstructible , and modifies the size of the map if it fails to find.

at throws if it fails to find.

find doesn't have the restriction of operator[] , and neither modifies nor throws if it fails to find.