在可变参数模板中使用sizeof运算符可在递归结束时跳过该函数

Question

I am trying to use operator sizeof... to skip the function for the end of recursion by not calling it if there are no argument

#include<iostream>

template<typename T, typename ...Types>
void Display(T firstArg, Types...Args)
{  
    std::cout << firstArg << "\n";
    std::cout << sizeof...(Types) << "\n";
    std::cout << sizeof...(Args) << "\n";
    if (sizeof...(Args) > 0)
    Display(Args...);
}

int main()
{
    Display(1, 2, 3,"hello");
    return 0;
}

But I am getting following error for Display(Args...);

error C2780: 'void Display(T,Types...)': expects 2 arguments - 0 provided

Workaround is to add function for the end of recursion (which I want to avoid)

void Display()
{

}

Question is how to avoid end of recursion function

Answer 1

You can't do this without some workaround, pre-C++17, unless you rewrite the function to not be recursive. The reason is that the entire function body is substituted, including branches of if statements that can never happen. That means that the compiler sees the call to Display() with no arguments, even though it would never have happened at runtime.

Since C++17, the solution to this is to use if constexpr instead of just if . That tells the compiler to evaluate the condition at compile time, and not try to compile the branch which doesn't execute.

Note that “not try to compile” above is a simplification; the link has more details about what exactly is and isn't done.