".size" 左边必须是类/结构
#include <iostream>
#include <string>
using namespace std;
class Sales_Data
{
public:
Sales_Data() = default;
Sales_Data(const string &s) :str_isbn(s) {}
const string& isbn()const { return str_isbn; }
private:
string str_isbn;
};
int main()
{
Sales_Data sd1{ "sss20170506" }, sd2{ "js" };
if (sd1.isbn().size() < sd2.isbn.size())
{
cout << "<" << endl;
}
else
{
cout << ">" << endl;
}
}