c++ 标准::map<std::string, int> 与 std::string_view 一起使用</std::string,>

Question

Suppose I have a std::map<std::string, int> . Is there any way to use its at method with std::string_view ? Here is a code snippet:

std::string_view key{ "a" };
std::map<std::string, int> tmp;
tmp["a"] = 0;
auto result = tmp.at(key);

Here is the output I have from clang 12.0

error: no matching member function for call to 'at'

auto result = tmp.at(key);

Answer 1

Three things are required for something like this to happen:

1. The map's comparator must be a transparent comparator (requires C++14, but you're already using string_view which is C++17, so this is a moot point).

2. The at() method must have an overload that participates in overload resolution when the container has a transparent comparator.

3. the parameter must be convertible to the map's key_type .

Neither of these are true in your example. The default std::less comparator is not a transparent comparator, there is no such overload for at() , and std::string does not have an implicit conversion from std::string_view .

There's nothing you can do about at() , however you can do something about the comparator namely using the (transparent std::void comparator) , and then use find() instead of at() , which does have a suitable overload:

#include <map>
#include <string>
#include <string_view>


int main()
{
    std::string_view key{ "a" };
    std::map<std::string, int, std::less<void>> tmp;
    tmp["a"] = 0;

    auto iter=tmp.find(key);
}

More complete demo

Answer 2

There is no implicit conversion from std::string_view to std::string , that is why you get a "no matching member function" error. See: Why is there no implicit conversion from std::string_view to std::string?

There is a std::string constructor that will accept a std::string_view as input, however it is marked as explicit , so you will have to do this instead:

auto result = tmp.at(std::string(key));

Demo

Same if you wanted to use the map's operator[] with std::string_view :

tmp[std::string(key)] = ...;
auto result = tmp[std::string(key)];