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Traits.cpp
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Traits.cpp
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///////////////////////////////////////////////////////////////////////////////
//
// Traits Class, Range-For Loop Iteration and decltype
//
// by Giovanni Dicanio <giovanni DOT dicanio@gmail.com>
//
// Code associated to the blog post:
// http://blogs.msmvps.com/gdicanio/2016/06/13/mixing-traits-classes-range-for-iteration-and-decltype-a-subtle-bug/
//
///////////////////////////////////////////////////////////////////////////////
#include <iostream> // For std::cout
#include <string> // For std::string
#include <type_traits> // For std::decay
#include <vector> // For std::vector
using namespace std; // Just fine in demo code
//-----------------------------------------------------------------------------
// Generic Traits Class
//-----------------------------------------------------------------------------
template <typename Type>
struct Traits
{
static int GetSomeProperty(const Type&)
{
cout << "Generic";
return 0;
}
//
// In production code there can be more public static methods here.
//
};
//-----------------------------------------------------------------------------
// Traits specialized for std::string
//-----------------------------------------------------------------------------
template <>
struct Traits<string>
{
static int GetSomeProperty(const string&)
{
cout << "String";
return 1;
}
};
//-----------------------------------------------------------------------------
// Test function iterating a generic container and querying some property
// on its elements using the above traits class.
//-----------------------------------------------------------------------------
template <typename Container>
void DoSomething(const Container & c)
{
for (const auto& elem : c)
{
cout << elem << ": ";
// Wrong result, since decltype(elem) is a "const string&", not a "string":
// const int x = Traits<decltype(elem)>::GetSomeProperty(elem);
// FIX:
// const int x = Traits<std::decay<decltype(elem)>::type>::GetSomeProperty(elem);
// FIX with better readability:
typedef std::decay<decltype(elem)>::type ElemType;
const int x = Traits<ElemType>::GetSomeProperty(elem);
cout << '\n';
}
}
//-----------------------------------------------------------------------------
// Test
//-----------------------------------------------------------------------------
int main()
{
vector<string> test{ "Hello", "Connie" };
DoSomething(test);
}
///////////////////////////////////////////////////////////////////////////////