c++ - C++ 中的模板断言？

Question

有没有办法定义模板

assertInheritsFrom<A, B>

这样

assertsInheritsFrom<A, B>

编译当且仅当

class A : public B { ... } // struct A is okay too

谢谢!

Answer 1

将静态断言与 is_base_of<Base,Derived> 相结合来自 Boost.TypeTraits:

BOOST_STATIC_ASSERT(boost::is_base_of<B, A>::value);

一个天真的实现(不考虑整数类型、私有(private)基类和歧义)可能如下所示:

template<class B, class D>
struct is_base_of {
    static yes test(const B&); // will be chosen if B is base of D
    static no  test(...);      // will be chosen otherwise
    static const D& helper();
    static const bool value = 
        sizeof(test(helper())) == sizeof(yes);
    // true if test(const B&) was chosen
};