直接覆盖map :: compare与lambda函数

问题描述

尝试使用lambda覆盖map::compare函数，看来以下解决方案有效.

Trying to override map::compare function using lambda, it seems that the following solution works.

auto cmp = [](const int&a, const int& b) { return a < b; };
std::map<int, int, decltype(cmp)> myMap(cmp);

但是，我必须先定义cmp，然后再使用.
我可以不定义"cmp"来做到这一点吗?

But, I had to define cmp first and use it later.
Can I do this without defining 'cmp'?

推荐答案

否，您不能在未评估的上下文中使用lambda，即示例中的模板参数.因此，您必须在其他地方定义它(使用auto)，然后使用decltype ...，相反，正如已经提到的那样，它是使用常规"函子

No, you can't use lambda in unevaluated context -- i.e. template parameters as in your example.So you must define it somewhere else (using auto) and then use decltype... the other way, as it was mentioned already is to use an "ordinal" functors

如果您的问题是关于"如何在定义地图时使用lambda表达式* * "，则可以利用lambda隐式转换为std::function的方法:

If your question is about "how to use lambda expression *once* when define a map" you can exploit implicit conversion of lambdas to std::function like this:

#include <iostream>
#include <functional>
#include <map>

int main()
{
    auto m = std::map<int, int, std::function<bool(const int&, const int&)>>{
        [](const int& a, const int& b)
        {
            return a < b;
        }
    };
    return 0;
}

您可以为该map类型引入别名，以减少以后的输入...

you may introduce an alias for that map type to reduce typing later...

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