<-- home

Learning concept checking the hard way - P3

March 25, 2014

In the previous posts this and this, we looked at the basics of concept checking and stuff you need to know.

Finally here’s our has_less method.

#include <iostream>
#include <vector>
#import <complex>

using namespace std;

template <typename T, typename unused = decltype(T() < T())>
std::true_type has_less_helper(const T&);

template <typename T>
std::false_type has_less_helper(...);

template <typename T>
constexpr bool has_less(void){
    using my_type = decltype(has_less_helper<T>(std::declval<T>()));
    return my_type::value;
}

template <typename T>
class BinarySearchTree{
public:
    static_assert(
        has_less<T>(), 
        "Assertion failed. No less than operation defined."
    );
    BinarySearchTree() {
    }
};

int main(int argc, const char * argv[])
{
    BinarySearchTree<int> binarySearchTree;
    // Compile time error here, since complex<int> doesn't
    // have a '<' operator.
    BinarySearchTree<complex<int>> compBinarySearchTree;
    return 0;
}

Let’s dissect the above method and see what’s happening.

has_helper method has two possible overload options. For the int datatype both are possible overload candidates but as we saw earlier compiler prefers the non-variadic option. So it choosses the first option which has a return type of std::true_type.

However for complex<int>, compiler has only one option since < doesn’t exist for complex type.

// we don't need to define the methods
// Since we are not calling them.
// we are just using the principle of SFINAE that applies to template
// arguments and not to the body

template <typename T, typename unused = decltype(T() < T())>
std::true_type has_less_helper(const T&);

//and

template <typename T>
std::false_type has_less_helper(...);

Note: true_type and false_type are just structs with values as true and false resp. Check out their usage

We could directly use has_less_helper to know what Type has a less than operator defined for it. However it is important to note that the has_less_helper assumes that type T has a default constructor defined for it(decltype(T() < T())).

To remove above dependency we could use declvalue(as discussed in previous post). That is what we are doing in has_less method.

template <typename T>
constexpr bool has_less(void){
    // using is equivalent to typedef
    using my_type = decltype(has_less_helper<T>(std::declval<T>()));
    // note that the return type is either true_type or false_type
    return my_type::value;
}

Note: the constexpr keyword makes the method evaluate at compile time. So we will have a true or false value from this method at compile time.

Inside our definition of generic BinarySearchTree we put a static_assert(compile time assert) to check that we have a less that operator define for the type T.

template <typename T>
class BinarySearchTree{
public:
    static_assert(
        has_less<T>(), 
        "Assertion failed. No less than operation defined."
    );
    BinarySearchTree() {
    }
};

That’s it! Now when you declare something like

BinarySearchTree< complex<int> > compBinarySearchTree;

We get a compile time error.

Awesome! Now you understand the concept of concept checking. Happy coding.