c++ - 折叠表达式和参数包 : difference betwen Args&& and Args inside static_assert

Question

这段代码来自
https://en.cppreference.com/w/cpp/language/fold

template<typename T, typename... Args>
void push_back_vec(std::vector<T>& v, Args&&... args){
    static_assert((std::is_constructible_v<T, Args&&> && ...));
    (v.push_back(std::forward<Args>(args)), ...);
}

我不明白为什么静态断言检查 Args&& 而不是 Args？

Answer 1

当您只有类型声明而没有定义时(std::is_constructible on incomplete types - 在这种情况下 is_constructible 是 UB )，这是针对这种特殊情况完成的:

struct B;

struct A {
    A(const A&) = default;
    A(const B&) {}

    A(A&&) = default;
    A() = default;
};
B&& getB();

template<typename T, typename... Args>
void push_back_vec(std::vector<T>& v, Args&&... args){
    static_assert((std::is_constructible_v<T, Args&&> && ...));
    (v.push_back(std::forward<Args>(args)), ...);
}

template<typename T, typename... Args>
void push_back_vec2(std::vector<T>& v, Args&&... args){
    static_assert((std::is_constructible_v<T, Args> && ...));
    (v.push_back(std::forward<Args>(args)), ...);
}

int main(int argc, char* argv[])
{
    std::vector<A> a;

    push_back_vec(a, A(), A());
    push_back_vec2(a, A(), A());

    A aa;
    push_back_vec(a, aa, aa, A());
    push_back_vec2(a, aa, aa, A());


    push_back_vec(a, getB());
    // code below won't compile
    //push_back_vec2(a, aa, A(), getB());
}

Live example