istringstream 運算符>> 返回值如何工作？

Question

此示例讀取具有 integer、運算符和另一個 integer 的行。 例如，

25 * 3

4 / 2

// sstream-line-input.cpp - Example of input string stream.
//          This accepts only lines with an int, a char, and an int.
// Fred Swartz 11 Aug 2003

#include <iostream>
#include <sstream>
#include <string>
using namespace std;
//================================================================ main
int main() {
    string s;                 // Where to store each line.
    int    a, b;              // Somewhere to put the ints.
    char   op;                // Where to save the char (an operator)
    istringstream instream;   // Declare an input string stream

    while (getline(cin, s)) { // Reads line into s
        instream.clear();     // Reset from possible previous errors.
        instream.str(s);      // Use s as source of input.
        if (instream >> a >> op >> b) {
            instream >> ws;        // Skip white space, if any.
            if (instream.eof()) {  // true if we're at end of string.
                cout << "OK." << endl;
            } else {
                cout << "BAD. Too much on the line." << endl;
            }
        } else {
            cout << "BAD: Didn't find the three items." << endl;
        }
    }
    return 0;
}

operator>>返回 object 本身 (*this)。

測試if (instream >> a >> op >> b)如何工作的？

我認為測試總是true ，因為instream!=NULL 。

Answer 1

basic_ios class （它是istream和ostream的基礎）具有到void*的轉換運算符，可以隱式轉換為bool 。 這就是它的工作原理。

Answer 2

“[15.4] 時髦的 while (std::cin >> foo) 語法是如何工作的？”