Why does the code work after adding scope resolution parameter?

Question

So I have this program that has a 3D point class and a function that finds out the distance between two points. When I use the distance function normally in the main, it gives an error. But when I add scope resolution operator, the code works. What is causing the error and how the scope resolution operator is fixing it?

This error occurs in Dev-C++ and Codeblocks but works fine with Atom IDE using gpp-compiler plugin with MinGW compiler.

#include <iostream>
#include <cmath>
using namespace std;

class Point{...} //Class object with x, y, and z variable and has functions to return values

float distance(Point p1, Point p2);

int main() {
    Point P1, P2;
    d = distance(P1, P2); // throws an error but just adding -> ::distance(P1, P2) works fine! why?
    cout << "Distance between P1 and P2: " << d << endl;
    return 0;
}

float distance(Point p1, Point p2) {
    float d;
    int x0 = p1.getX(), y0 = p1.getY(), z0 = p1.getZ();
    int x1 = p2.getX(), y1 = p2.getY(), z1 = p2.getZ();
    d = sqrt(pow((x1-x0),2) + pow((y1-y0), 2) + pow((z1-z0), 2));
    return d;
}

Possible duplicate: https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice — 463035818_is_not_an_ai, May 14 '19 at 09:43
Short answer: https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice — lisyarus, May 14 '19 at 09:44
By the way, you did not initialize ```P1``` and ```P2``` in your main. — Croppi, May 14 '19 at 09:44
Might this be a question of namespace? There might be a `std::distance` function. — , May 14 '19 at 09:44
@Croppi more is missing from this snippet. @ OP please include a [mcve] and the error message — 463035818_is_not_an_ai, May 14 '19 at 09:45
@generic_opto_guy [there is](https://en.cppreference.com/w/cpp/iterator/distance) — Mike van Dyke, May 14 '19 at 09:45
Maybe a compiler bug. Which version of Dev-C++, although they are all pretty old at this point? On Windows Visual Studio is free, or you can make sure you have a recent version of MinGW/GCC directly for a linux-like environment. While there is an `std::distance`, and you used `std`, I do not believe it should cause a problem with you declaring `distance` that way (quickly checked VS2019 and GCC 5.4). — Fire Lancer, May 14 '19 at 09:48
@FireLancer given the absence of a mcve its a bit far-fetched to speculate a compiler error imho. I mean we dont even know what error is reported — 463035818_is_not_an_ai, May 14 '19 at 09:50
@formerlyknownas_463035818 We have the code though, and that code should work fine, and does on any recent GCC or VS I tried (given a dummy `Point` etc. needed for `::distance` as well), even if I added `` intentionally. `std::distance` is a template function, and he declared `float distance(Point p1, Point p2)` before he tried to use it. — Fire Lancer, May 14 '19 at 09:51
@FireLancer we have code but it is incomplete and sprinkled with syntax errors, and we have no information on what the error actually says.... — 463035818_is_not_an_ai, May 14 '19 at 09:54

score 1 · Accepted Answer · answered May 14 '19 at 09:46

1

It's impossible to tell for sure without the actual error message, but it looks like the problem is using namespace std;. This brings in the function std::distance() which isn't what you want, but using the scope operator to request the global distance() function works again.

Avoid bringing in the entire std namespace.

answered May 14 '19 at 09:46

Harun

1,582
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2

It shouldn't try to use `std::distance` in the code as posted, even if `` is included. `float distance(Point p1, Point p2)` should be preferred over the template version. – Fire Lancer May 14 '19 at 09:50
std::distance is defined in and not – Shriram May 14 '19 at 09:54
3

@Shriram There is no reason why couldn't include , standard does not disallow including additional files – VLL May 14 '19 at 09:58

Shriram · Answer 2 · 2019-05-14T10:06:27.810

So I compiled your code in Visual Studio 2019 and a couple of online compilers and it works just fine. I assumed a few things but honestly adding a :: shouldn't affect anything in this case.

also , I couldn't find where you declared float d, It would really clear things up if you showed us the error message!

#include <iostream>
#include <cmath>
using namespace std;

class Point {
    float x=0.f, y=0.f, z=0.f;
public:
    Point() {}
    Point(float x1, float y1, float z1) :x(x1), y(y1), z(z1) {}
    float getX() const{ return x; }
    float getY() const{ return y; }
    float getZ() const{ return z; }
};//Class object with x, y, and z variable and has functions to return values

float distance(Point p1, Point p2);

int main() {
    Point P1, P2;
    float d = distance(P1, P2); // throws an error but just adding -> ::distance(P1, P2) works fine! why?
    cout << "Distance between P1 and P2: " << d << endl;
    return 0;
}

float distance(Point p1, Point p2) {
    float d;
    int x0 = p1.getX(), y0 = p1.getY(), z0 = p1.getZ();
    int x1 = p2.getX(), y1 = p2.getY(), z1 = p2.getZ();
    d = sqrt(pow((x1 - x0), 2) + pow((y1 - y0), 2) + pow((z1 - z0), 2));
    return d;
}

Why does the code work after adding scope resolution parameter?

2 Answers2