C++ 变量检查

Question

假设我希望用户输入一个整数，但他输入了 double 值或字符值，我该如何检查用户是否输入了正确的类型。

string line;
getline(cin, line);
// create stringstream object called lineStream for parsing:
stringstream lineStream(line); 
string rname;
double res;
int node1,node2;
lineStream >> rname >> res >> node1 >> node2;

如何检查有效的输入类型？

Answer 1

您检查流是否正常:

if (lineStream >> rname >> res >> node1 >> node2)
{
    // all reads worked.
}

你可能想在最后检查垃圾。

if (lineStream >> rname >> res >> node1 >> node2)
{
    char x;
    if (lineStream >> x)
    {
         // If you can read one more character there is junk on the end.
         // This is probably an error. So in this situation you
         // need to do somethings to correct for this.
         exit(1);
    }

    // all reads worked.
    // AND there is no junk on the end of the line.
}

评论展开。

来自以下评论:

if i input an integer for rname, it still works. for exmaple:

string line; getline(cin, line);
stringstream lineStream(line); // created stringstream object called lineStream for parsing
string rname; 
if (lineStream >> rname) { cout << "works"; }

让我们假设 rname 有一些属性可以让我们将它与数字区分开来。例如:它必须是一个名字。即它必须只包含字母字符。

struct Name
{
    std::string   value;
    friend std::istream& operator>>(std::istream& s, Name& data)
    {
        // Read a word
        s >> data.value;

        // Check to make sure that value is only alpha()
        if (find_if(data.value.begin(), data.value.end(), [](char c){return !isalpha(c);}) != str.end())
        {
            // failure is set in the stream.
            s.setstate(std::ios::failbit);
        }
        // return the stream
        return s;
    }
};

现在你可以读一个名字了。

Name rname; 
if (lineStream >> rname) { cout << "works"; }

如果您为 rname 输入一个整数，这将失败。

拉伸(stretch)答案

如果您有多行相同的信息要阅读。然后值得将它包装在一个类中并定义一个输入流运算符。

strcut Node
{
    Name   rname;
    double res;
    int    node1;
    int    node2;

    friend std::istream& operator>>(std::istream& s, Node& data)
    {
        std::string line;
        std::getline(s, line);

        std::stringstream linestream(line);
        lineStream >> data.rname >> data.res >> data.node1 >> data.node2;

        if(!linestream)
        {
            // Read failed.
            s.setstate(std::ios::failbit);
        }
        return s;
    }
};

现在可以很容易地阅读循环中的行:

Node n;
while(std::cin >> n)
{
    // We read another node successfully
}