What is the underscore before the parentheses do in C++?

Question

I am reading the following C++17 code:

#include <iostream>
#include <cstdio>
#include <string>
#include <vector>
#include <fstream>
#include <tuple>
using namespace std;

enum class Relationship { parent, child, sibling };
struct Person { string name; };
struct Relationships
{
    vector<tuple<Person, Relationship, Person>> relations;
    void add_parent_and_child(const Person& parent, const Person& child)
    {
        relations.push_back({parent, Relationship::parent, child});
        relations.push_back({child, Relationship::child, parent});
    }
};

struct Research
{
    Research (Relationships& relationships)
    {
        auto& relations = relationships.relations;
        for(auto&& [first, rel, second] : relations)
        {
            if(first.name == "John" && rel == Relationship::parent)
            {
                cout << "John has a child called "  << second.name << endl;
            }
        }
    }
};

int main()
{
    Person parent {"John"};
    Person child1 {"Chris"}, child2 {"Matt"};
    Relationships relationships;
    relationships.add_parent_and_child(parent, child1);
    relationships.add_parent_and_child(parent, child2);

    Research _(relationships);

    return 0;
}

That code compiles and works fine. What I do not get is the line:

    Research _(relationships);

Why is there an underscore? Is this a new feature of C++17? What is it called?

https://stackoverflow.com/questions/18646935/is-single-underscore-a-valid-c-variable-name maybe this one? — ik1ne, Sep 14 '19 at 04:16

score 0 · Answer 1 · edited Mar 23 '20 at 23:27

0

Wow, it turns out to be a variable name, valid in C++17. That's a very lazy way of naming things.

edited Mar 23 '20 at 23:27

halfer

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answered Sep 14 '19 at 04:21

J. Doe

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It’s not necessarily lazy—sometimes it means that the name doesn’t matter because the variable exists for the effects of creating/destroying it. See the duplicate for yet other possible reasons. – Davis Herring Mar 24 '20 at 20:38

What is the underscore before the parentheses do in C++?

1 Answers1