C++ istringsteam 是如何工作的？

Question

对于我的家庭作业，我使用 std::istringstream 以便在适用时创建错误消息。

我的程序工作正常，除了两种特定情况，它不能正常工作。

第一个测试用例是第二个数字为零且第一个数字大于零的数字。

第二个测试用例是第二个数字 n 大于第一个数字 m。

n > m 时应该 return 错误信息，但它直接得出结果。

我的问题是，std:::istringstream 究竟是如何工作的？它如何比较这一行 argv 中的值？我怎样才能修复我的代码以便它检查正确的东西？

更具体地说，这条线是做什么的？

(!(iss >> m))

我的老师在较早的实验中给了我们这个 std::istringstream 代码，但我认为它只检查正数，这就是它在我包含的第一个测试用例中失败的原因。我该如何解决这个问题？

int main(int argc, char* argv[])
{
    unsigned int m;
    unsigned int n;
    //error checks
    istringstream iss;
    if(argc != 3) {
        cerr<< "Usage: " << argv[0] << " <integer m> <integer n>" << endl;
        return 1;
    }
    iss.str(argv[1]);
    if (!(iss >> m) ){
        cerr << "Error: The first argument is not a valid nonnegative integer." << endl;
        return 1;
    }

    iss.clear();
    iss.str(argv[2]);
    if (!(iss >> n) ){
        cerr << "Error: The second argument is not a valid nonnegative integer." << endl;
        return 1;
    }

    if (n > m){
        cerr << "Error: The second argument is not a valid nonnegative integer." << endl;
        return 1;
    }

    //correct output
    cout << m << " x " << n << " = " << russian_peasant_multiplication(m, n) << endl;
    return 0;
}

Answer 1

此代码不检查 non-negative 整数，希望这能说明问题。我将使用本地数组 char* args[3] 作为参数值。:

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main(int argc, char* argv[])
{
    unsigned int m;
    unsigned int n;
    argc = 3;
    const char* args[3];
    args[1] = "-1";
    args[2] = "-2";

    istringstream iss;
    if (argc != 3) // Passes because argument count is 3 
    {
        cerr << "Usage: " << argv[0] << " <integer m> <integer n>" << endl;
        return 1;
    }

    iss.str(args[1]); // iss now holds "-1"

                      // istringstream operator >> will set either eofbit, failbit or badbit
                      // on failure. The iss object is convertible to bool so you can check
                      // in an if statement.

    if (!(iss >> m))  // Extracts "-1" into unsigned int m.
                      // m holds 4294967295 because it's unsigned.
                      // if statement doesn't fail because istringstream can 
                      // can extract into signed ints also
    {
        cerr << "Error: The first argument is not a valid nonnegative integer." << endl;
        return 1;
    }

    iss.clear();
    iss.str(args[2]); // iss object now holds "-2"
    if (!(iss >> n))  // Same here, extraction into unsigned in doesn't fail
    {
        cerr << "Error: The second argument is not a valid nonnegative integer." << endl;
        return 1;
    }

    if (n > m) // This checks if n > m, but still nowhere in the code have you
               // checked if the argv[] values are negative or positive values.
    {
        cerr << "Error: The second argument is not a valid nonnegative integer." << endl;
        return 1;
    }

    cout << m << " x " << n << " = " << endl;
    return 0;
}

C++ istringsteam 是如何工作的？

C++ How does istringsteam work?

c++

istringstream