如何设计一个可以轻松返回数组长度的函数？

Question

I want to design a function that can return the value of an array easily. But I meet some problems. In the return statement of function getArrayLen() , the value of sizeof(array) seems to be 4, which is obviously wrong. But if I use sizeof(people) in the main function, it returns 300, which is right. I wonder why? I konw that sizeof(array) returns 4 because "array" is a pointer, but isn't "people" in sizeof(people) a pointer, too?

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

template <class T>
int getArrayLen(T& array)
{
    cout << "size of array: " << sizeof(array) << endl;
    cout << "size of array[0]: " << sizeof(array[0]);
    return (sizeof(array) / sizeof(array[0]));
}

struct Person
{
    string name;
    int age;
    string sex;
};

void test(Person arr[])
{
    int length = getArrayLen(arr);
}

int main() 
{
    Person people[5] =
    {
        {"John",23,"Male"},
        {"Alan",22,"Male"},
        {"Alex",20,"Male"},
        {"James",21,"Male"},
        {"Lucy",19,"Male"}
    };
    test(people);

    return 0;
}

Answer 1

How can I design a function that can return the length of an array easily?

You could use a function template that deduces the size from an array argument. However, there is no need to design such template, because standard library provides one already: std::size .

the value of sizeof(array) seems to be 4, which is obviously wrong. But if I use sizeof(people) in the main function, it returns 300, which is right. I wonder why?

That is because array , despite its name, and despite what the declaration looks like, is not an array. A function parameter is never an array.

The parameter is a pointer which happens to an element of an array. Your function cannot be used to get the size of an array using a pointer to an element of the array.

but isn't "people" in sizeof(people) a pointer, too?

No. people is not a pointer. It is an array.

Answer 2

Just fix the functions: getArrayLen & test to general function:

    template <class T>
    int getArrayLen(const T& array) {
        return sizeof(T);
    }

    template <class T>
    void test(const T& array)
    {
        int length = getArrayLen(array);
        cout << length << endl;
    }

Result:

    300