C++ 有免费的函数`size(object)`吗?

It seems that the way that most people find the size of a string is they just use the my_string.size() and it works fine. Well, I recently did an assignment for class where I did...

if (size(my_string) < 5)
    store[counter].setWeight(stoi(my_string));

而不是......

if (my_string.size() < 5)
    store[counter].setWeight(stoi(my_string));

但令我惊讶的是，我认为正在运行旧编译器的讲师无法运行该行代码.在我的编译器上，它是双向的，我不太确定为什么.

#include <string>
#include <iostream>
using namespace std;

int main()
{
    string myvar = "1000";
    cout << "Using size(myvar) = " << size(myvar) << endl;
    cout << "Using myvar.size() = " << myvar.size() << endl;
}

template<class _Container>
auto inline size(const _Container& _Cont)
    -> decltype(_Cont.size())
{   // get size() for container
return (_Cont.size());
}

cout << "Using size(myvar) = " << size(myvar) << endl;