CODEWITHSUNDEEP
c++c++17fold-expression
Nick
Nick

Reputation: 10539

Fold expressions and parameter pack: difference betwen Args&& and Args inside static_assert

This code is from
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)), ...);
}

I do not understand why static assert checks for Args&& and not for Args?

Upvotes: 2

Views: 114

Answers (1)

bartop
bartop

Reputation: 10315

This is done for corner cases like this one, when you have only type declaration but no definition (std::is_constructible on incomplete types - in which case is_constructible is UB ):

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

Upvotes: 1

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