Visual studio unable to resolve overloaded unambiguous function in class and namespace - c++

Question

See code below
#include <iostream>
#include <string>
namespace stringhelper
{
std::string to_string(int n) { return "0"; } // ignore wrong implementation. simplified for example purpose
}
using stringhelper::to_string;
class TestClass
{
public:
std::string to_string() const { return "TestClass:" + to_string(m_value); }
private:
int m_value;
};
int main()
{
TestClass tc;
std::cout << tc.to_string();
}
If TestClass does not implement function to_string(), within TestClass, it is able to resolve to_string(m_value) to stringhelper::to_string(int). However, the moment TestClass implements function to_string(), the compiler is unable to resolve to_string(int) to stringhelper::to_string.
Rather, it insists/resolves the function to TestClass::to_string and gave an error that the function TestClass::to_string does not take in 1 arguments.
Why is this so?
Environment:
Visual Studio 2008 Professional Edition 9.0.21022.8 RTM
Configuration: Win32
Windows 8

Answer 1

This behavior is not limited to Visual Studio 2008. If tested in modern Clang implementations you will see the same behaviour. As you may know, functions in derived classes which don't override functions in base classes but which have the same name will hide other functions of the same name in the base class.
The "problem" here is that you, by using the using statement introduces a function named to_string into a scope that is essentially a victim of the exact same thing as what happens in the above example, when looking at it from inside your class.
If the standard had you call member functions with this->foo() this would probably not have been an issue. But since function calls within a class are presumed to be part of the class and only if not found looked for in other scopes this becomes an issue.
Since you have an implementation in your class, that has priority and will be used. Since you want a version that takes an int as an argument, an overloaded version of your member function will be looked for and since it does not exist you get the error you see.
This is part of why using namespace can often introduce errors that might be unintuitive to understand. If you want to make sure you use the stringhelper::to_string implementation while you are in a class with a function that has the same name you have to be explicit.
This would work fine for instance, even if you keep your using statement.
#include <iostream>
#include <string>
namespace stringhelper
{
std::string to_string(int n) { return "0"; } // ignore wrong implementation. simplified for example purpose
}
using stringhelper::to_string;
class TestClass
{
public:
std::string to_string() const { return "TestClass:" + stringhelper::to_string(m_value); }
private:
int m_value;
};
int main()
{
TestClass tc;
std::cout << tc.to_string();
}