Question

Consequently to a previous question about const/non-const with ternary operator, is the following test function ok regarding to the C++11 standard :

template<bool UseConst> class MyClass
{
    public:
        constexpr bool test()
        {
             return (UseConst) ? (_constvar) : (_var);
        }

    protected:
        int _var;
        static const int _constvar;
}

The whole problem, is that _constvar is const, and _var is non-const. I would have to access these 2 data depending on the template parameter through the same function, and I would like to have a compile-time function when I use const.

Do the test() function satisfy my requirements ?

Answer 1

You could use SFINAE in order to “specialize” your test function. In other words, you could do something like the following:

template<bool true_false>
struct true_type
{
    static char value;
};

template<>
struct true_type<false>
{
    static char value[2];
};

template<bool UseConst> class MyClass
{
    private:
        constexpr int pre_test(const char arg) { return _constvar; }
        int pre_test(char (&)[2]) const { return _var; }

    public:
        constexpr int test()
        {
             return pre_test(true_type<UseConst>::value);
        }

    protected:
        int _var;
        static const int _constvar;
};

Now, when you call MyClass::test, if UseConst is false, test will degrade to a run-time function, but when UseConst is true, you will get a compile-time function.