模板 class 无实例构造函数

Question

I want to be able to make my Wrapper shorter.

I dont want to have to include decltype(foo) twice in each line

What is the right way to use the constructor to be able yo call _foo2 like im trying.

class Helper {
public:
    Helper(LPCTSTR filename) : _filename(filename) {}
    ~Helper() { }

    template<typename T>
    T CallFunction() {
        return T();
    }

private:
    LPCTSTR _filename;
};

template<typename T> class WrapFuncObj;
template<typename T, typename... Args>
class WrapFuncObj<T(Args...)> {
public:
    WrapFuncObj(Helper* dll, LPCTSTR func) : _dll(dll), _func(func) {}

    T operator()(Args&&... args) {
        return _dll->CallFunction<T>();
    }

private:
    Helper* _dll;
    LPCTSTR _func;
};


class Export {
    Helper _dll{ L"SayHello.dll" };

public:
    WrapFuncObj<decltype(foo)> _foo = WrapFuncObj<decltype(foo)>(&_dll, L"foo"); // works
    WrapFuncObj<decltype(foo)> _foo2 = WrapFuncObj(&_dll, L"foo"); // problem here
};

Answer 1

You can provide the constructor arguments directly, no need to indirect through another WrapFuncObj functional cast. It works the same way as you used it for _dll :

WrapFuncObj<decltype(foo)> _foo2{&_dll, L"foo"};

Just be aware that initialization with parentheses is not allowed for default initializers of data members, you need to use braces.