I am trying to create a priority_queue with each element as a 3D vector. The element whose third dimension has the largest value would become the priority element.
Here is my code:
priority_queue< vector<int>, vector<vector<int>>, [](vector<int> &v1, vector<int> &v2){
return v1[2] > v2[2];
}> pq{};
but I got the following error:
error: lambda-expression in template-argument
Any idea what I did wrong? Thanks!
You cant have a lambda (which is an object) in a type template declaration (where a type is needed. Do instead:
auto lambda = [](vector<int> &v1, vector<int> &v2){
return v1[2] > v2[2];
};
priority_queue< vector<int>, vector<vector<int>>,decltype(lambda)> pq{lambda};
We also need to pass the lambda since it is not default constructible.
Form C++20 on, we can do the following:
priority_queue< vector<int>, vector<vector<int>>,decltype([](vector<int> &v1, vector<int> &v2){
return v1[2] > v2[2];
};
)> pq{};
Here there come two new featurs of lambdas into play. First that you can take the type of it directly (formally: In C++17 you cannot have a lambda-expression in unevaluated context) and second that lambdas are default constructible. But right now, that is not yet possible.
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