What is name lookup mechanism?

Question

I\'d like to know what C++ name lookup mechanism is.

Answer 1

I don't know whether you ask for an analogy to describe what name lookup is. But I will give a possible answer to the above question. Before looking deeply into name lookup mechanism, let's mapping two concepts for fun and use.

1. scope -> directory
2. object -> file

I don't want to explain the reason for this analogy. In the following literal way of thinking, replace scope by directory every time scope is encountered. So does object by file.

#include 

using namespace std;

namespace Newton{
    double distance, time;
    double velocity(const double &, const double &);
}
namespace Einstein{
    double distance, time;
    double velocity(const double &, const double &);
}

int main()
{
    using namespace Newton;
    double s(10), t(10);
    velocity(s,t);
    return 0;
}

double Newton::velocity(const double & s, const double & t){
    distance = s;
    time = t;
    cout << "Calculation by Newton" << endl;
    return distance / time;
}

double Einstein::velocity(const double & s, const double & t){
    distance = s;
    time = t;
    cout << "Calculation by Einstein" << endl;
    return distance / time;
}

in this code, what is the implementation of velocity? We meet velocity funciton in the socpe of main(). If you name it like a path name in the file explorer, velocity is actually /Main/velocity. Of course, if you check object names under /Main/, there are still two double objects, s and t, and one namespace object, Newton. If you list the names of objects under /Main/double/, I think the built-in function does not matches an object named velocity, which means, for example, there is no such object -- /Built-in Types/double/velocity. If you list the names of objects again under /Main/Newton/, the actual directory is search is /Newton/ because it is declared there. Then, list the object names under /Newton/, we find two double objects, distance and time, and one function named velocity. Yes, we find a candidate function for /Main/velocity.

I can only give an analogy of name lookup in c++. There is more to add to conclude a mechanism.