Question

I have a class A that has a template parameter T. There are use cases where the class T offers a function func1() and there are use cases where T doesn t offer it. A function f() in A should call func1(), iff it exists. I think this should be possible with boost mpl, but I don t know how. Here some pseudo code:

template<class T>
class A
{
    void f(T param)
    {
        if(T::func1 is an existing function)
            param.func1();
    }
};

更糟的是:

template<class T>
class A
{
    void f(T param)
    {
        if(T::func1 is an existing function)
            param.func1();
        else
            cout << "func1 doesn t exist" << endl;
    }
};

Answer 1

Boost.MPL doesn t deal with that as it s strictly for TMP and you can t call members in TMP. Boost.Fusion and Boost.TypeTraits don t have anything either; I thought one of them would but apparently I m misremembering.

https://stackoverflow.com/questions/257288/is-it-possible-to-write-a-c-template-to-check-for-a-Functions- There/264088#264088>Here和here<> 关于如何在C++03中写一个海峡以检测一名成员,有些解决办法。一旦具备这种特性(请上has_func1_member),你可以将其用于SFINAE:

template<typename T>
typename boost::enable_if<has_func1_member<T> >::type
maybe_call(T& t)
{ t.func1(); }

template<typename T>
typename boost::disable_if<has_func1_member<T> >::type
maybe_call(T&)
{
    // handle missing member case
}

// your example code would then do:
maybe_call(param);

请注意,C++11虽然仍然有一些弧度,但首先更容易书写海峡。

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