Why do I get an error in this code when using "using namespace std;" and "bits/stdc++.h"?

Question

Actually this code works fine in "DEV C++", but when I put it into my "Hacker-Rank" panel it gives this error "reference to function is ambiguous", although all the online compilers are giving errors...

I don't think here function overloading is somewhere interrupting, because this error mostly comes in function overloading.

#include <bits/stdc++.h>
#include <cstdio>
#include<iostream>

using namespace std;


int function(int n);

int main()
{
    int n;
    cin >> n;
    cin.ignore(numeric_limits<streamsize>::max(), '\n');    

    if(n<=0){
        return(0);
    }
    else{
        function(n);
    }

}
int function(int n)
{
    if (n<=9)
    {
        cout<<"experiment";
    }
    
    else{
        cout<<"Greater than 9";
    }
    return 0;
}

The error with clang is:

<source>:20:9: error: reference to 'function' is ambiguous
        function(n);
        ^
<source>:8:5: note: candidate found by name lookup is 'function'
int function(int n);
    ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/bits/std_function.h:111:11: note: candidate found by name lookup is 'std::function'
    class function;
          ^
// ... and more ....

Answer 1

For starters this else code block

else{
    function(n);
}

returns nothing.

Though it is allowed but confuses readers of the program because they expect that if there is an explicit return statement in the if sub-statement then a similar return statement should be in the else sub-statement.

It seems the name function declared in the global name space conflicts with the standard name std::function due to the using directive.

using namespace std;

Write

else{
    return ::function(n);
}

Answer 2

The problem is caused by #include <bits/stdc++.h> combined with the directive using namespace std.

<bits/stdc++.h> includes most (all, depending on the age of the version you have with your compiler) headers related to the C++ standard library.

One of the headers included by <bits/stdc++.h> (since C++11) is <functional>, which declares a templated class std::function. std::function has a templated constructor that can accept a single argument of any type.

In your main(), anything declared (visible to the compiler) named function is a candidate for being used by the statement function(n). The directive using namespace std tells the compiler to consider names within std as candidates. According to rules of the language, both your declared function() and std::function are equally good matches for the name function.

The real fix has two parts. The first is to avoid using headers like <bits/stdc++.h> and, instead, only include standard headers that are actually needed by your program.

The second part is to avoid using the directive using namespace std excessively, or even at all. It can cause names (of types, functions, variables, etc) within standard headers to accidentally match names in your code.

If you do a search, you will find plenty of explanations of why to avoid both <bits/stdc++.h> and to avoid using namespace std (or other using directives). Both have their uses, but both introduce hard-to-avoid gotchas (such as you have experienced).

Answer 3

Vlad has shown that working around the problems caused by using namespace std; can solve your problem. That is a good answer.

Funnily, you can also fix your problem by not applying the antipattern #include <bits/stdc++.h>. Even without Vlads proposed improvement.

#include <limits>
#include <cstdio>
#include<iostream>


using namespace std;

int function(int n);

int main()
{
    int n;
    cin >> n;
    cin.ignore(numeric_limits<streamsize>::max(), '\n');    

    if(n<=0){
        return(0);
    }
    else{
        function(n);
    }

}
int function(int n)
{
    if (n<=9)
    {
        cout<<"experiment";
    }
    
    else{
        cout<<"Greater than 9";
    }
    return 0;
}

More info on why I dare to describe that as "antipattern" is here:
Why should I not #include <bits/stdc++.h>?