gets_s()函数不能用吗？一直错误，我在VS上为啥能用

#include<iostream>
#include<cmath>
#include<cstring>
using namespace std;
int f1(char a[][50],int n) {
	int st, count = 0;
	if (a[n][0] == 'a' || a[n][0] == 'b' || a[n][0] == 'c')st = 2;
	else st = 0;
	for (unsigned int i = st; i < strlen(a[n]); i++) {
		if (a[n][i] != ' ')count++;
		else break;
	}
	return count;
}
int f2(char a[][50], int n) {
	int st, count = 0, c2 = 0;
	if (a[n][0] == 'a' || a[n][0] == 'b' || a[n][0] == 'c')st = 2;
	else st = 0;
	for (unsigned int i = st; i < strlen(a[n]); i++) {
		if (a[n][i] == ' ')c2++;
		if (c2)count++;
		else continue;
	}
	return --count;
}
int f3(int n) {
	int count = 1;
	if (n < 0)n = -n;
	while (n >= 10) {
		n /= 10;
		count++;
	}
	return count;
}
int st(char a[][50], int n) {
	if (a[n][0] == 'a' || a[n][0] == 'b' || a[n][0] == 'c')return 2;
	else return 0;
}
int num[50][2];
int main() {
	int n;
	cin >> n;
	char choose;
	char str[50][50];
	for (int i = 1; i < n+1; i++) {
		gets_s(str[i]);
	}
	for (int i = 1; i < n+1; i++) {
		int co = f1(str,i);
		for (int j = st(str, i); j < st(str, i) + f1(str, i); j++) {
			num[i][0] += (int(str[i][j])-int('0')) * pow(10, co-1);
			co--;
		}
		co = f2(str,i);
		for (unsigned int j = st(str, i) + f1(str, i) + 1; j < strlen(str[i]); j++) {
			num[i][1] += (int(str[i][j])-int('0')) * pow(10, co-1);
			co--;
		}
	}
	choose = str[1][0];
	for (int i = 1; i < n+1; i++) {
		if (str[i][0] >= 97 && str[i][0] <= 122)choose = str[i][0];
		switch (choose) {
		case 'a':cout << num[i][0] << "+" << num[i][1] << "=" << num[i][0] + num[i][1] << endl;
			cout << f1(str, i) + f2(str, i) + f3(num[i][0] + num[i][1]) + 2 << endl;
			break;
		case 'b':cout << num[i][0] << "-" << num[i][1] << "=" << num[i][0] - num[i][1] << endl;
			cout << f1(str, i) + f2(str, i) + f3(num[i][0] - num[i][1]) + 2 << endl;
			break;
		case 'c':cout << num[i][0] << "*" << num[i][1] << "=" << num[i][0] * num[i][1] << endl;
			cout << f1(str, i) + f2(str, i) + f3(num[i][0] * num[i][1]) + 2 << endl;
			break;
		}
	}
}