c++11 可变参数模板编译失败

Question

只需几行代码，我希望“count”函数能够给出参数的个数:

#include <cstddef>
#include <utility>
using namespace std;
template<class T>
size_t f(T&& ... elem){
    return sizeof...(elem);
}
int main(){
    return 0;
}

但是编译失败

main.cpp:5:22: error: expansion pattern 'T&&' contains no argument packs
size_t count(T&& ... elem){
                    ^~~~
main.cpp: In function 'size_t count()':
main.cpp:6:22: error: 'elem' has not been declared
    return sizeof...(elem);
                    ^~~~
main.cpp: In function 'int main()':
main.cpp:9:23: error: no matching function for call to 'count(int, int, int)'
    return count(1,2,3);
                    ^
main.cpp:5:8: note: candidate: 'template<class T> size_t count()'
size_t count(T&& ... elem){
        ^~~~~
main.cpp:5:8: note:   template argument deduction/substitution failed:
main.cpp:9:23: note:   candidate expects 0 arguments, 3 provided
    return count(1,2,3);
                    ^


g++ -std=c++17 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
Compile, link and run...
Share!

如何解决？

Answer 1

还需要用...引入模板参数包:

template<class... T>         // declares the pack "T"
//            ^^^
size_t f(T&&... elem)        // declares the pack "elem", uses the pack "T"
{
    return sizeof...(elem);  // ("sizeof..." is an operator)
}