使用std :: apply应用可变参数函数(Applying a variadic function with std::apply)

 是否可以使用std :: apply将可变参数应用于元组？ 
 
 例如，下面的代码在GCC 6.2.1中可以正常工作： 
 
void print_t(std::string i, std::string j) {
    std::cout << i << " " << j << std::endl;
}

int main() {
        std::tuple<std::string, std::string> t{"ab", "cd"};
        std::experimental::apply(print_t, t);
        return 0;
}
 
 但是，如果我尝试应用可变参数函数： 
 
template<typename T>
void vprint(T && t) {
    std::cout << std::forward<T>(t) << std::endl;
}

template<typename T, typename ... Ts>
void vprint(T && t, Ts ... ts) {
    std::cout << std::forward<T>(t) << " ";
    vprint<Ts...>(std::forward<Ts>(ts)...);
}

int main() {
        std::tuple<std::string, std::string> t{"fd", "ab"};
        std::experimental::apply(vprint, t);
        return 0;
}
 
 编译器会抱怨它无法推导出vprint模板参数。 好的，让我们明确地写出来： 
 
std::experimental::apply(vprint<std::string, std::string>, t);
 
 现在编译器结束了一些暴露标准库内部的晦涩错误。 
 
 我在C ++ 11中编写了自己的std::apply实现，并且理解为什么它不能推导出可变参数函数模板的参数。 但是， 从理论上讲 ， std::apply具有所有扣除所需的信息。 
 
 那么在GCC6中，可变参数函数的应用还没有实现？ C ++ 17兼容的编译器是否允许这样的应用程序？ 如果没有，他们是否允许应用实例化的可变参数模板函数，如vprint<std::string, std::string> ？ 

Is it possible to apply a variadic function to a tuple with std::apply?
 
For example, the following code works fine with GCC 6.2.1:
 
void print_t(std::string i, std::string j) {
    std::cout << i << " " << j << std::endl;
}

int main() {
        std::tuple<std::string, std::string> t{"ab", "cd"};
        std::experimental::apply(print_t, t);
        return 0;
}
 
But if I try to apply a variadic function:
 
template<typename T>
void vprint(T && t) {
    std::cout << std::forward<T>(t) << std::endl;
}

template<typename T, typename ... Ts>
void vprint(T && t, Ts ... ts) {
    std::cout << std::forward<T>(t) << " ";
    vprint<Ts...>(std::forward<Ts>(ts)...);
}

int main() {
        std::tuple<std::string, std::string> t{"fd", "ab"};
        std::experimental::apply(vprint, t);
        return 0;
}
 
the compiler complains that it cannot deduce template arguments of vprint. OK, let's write them explicitly:
 
std::experimental::apply(vprint<std::string, std::string>, t);
 
Now the compiler ends up with some obscure errors which expose standard library internals.
 
I wrote my own implementation of std::apply in C++11 and I understand why it can't deduce arguments of the variadic function template. But, in theory, std::apply has all the information needed for that deduction.
 
So is the application of variadic functions a not yet implemented feature in GCC6? Will C++17-compatible compilers allow such application? If not, will they allow application of instantiated variadic template functions, like vprint<std::string, std::string>?

原文：https://stackoverflow.com/questions/40523334