家庭作业问题中的警告C++

Question

当我运行这个程序时得到两个警告,我无法弄清楚如何阻止这种情况发生.任何帮助,将不胜感激!

警告:有符号和无符号整数表达式之间的比较

是我在extract_word函数中获得2行的警告.

#include<iostream>
#include<string>

using namespace std;

class StringModify {

    public:

    void get_data();
    //takes a line of input from the user

    string& extract_word(string& a);
    //extracts each word from the input line

    string& reverse_word(string& s);
    //returns a string which is reverse of s

    void rev();

    void swap(char& v1, char& v2);
    //interchanges value of v1 and v2

    void append(const string& reverse_word);
    //puts together each reversed word with whitespaces to get formatted_line

    void display();
    //diss both input line and formatted_line

    private:
    string origional_string;   //original string
    string formatted_string; //formatted string
    string word;

};


int main() {
    StringModify data1;
    //data1 becomes the call for class StringModify

    data1.get_data();
    // Exicution of get_data in class StringModify
    data1.rev();
    // Exicution of rev in class StringModify
    data1.display();
    // Exicution of display in class StringModify

    return 0; }

void StringModify::get_data() {
    cout << "Enter the string: ";
    getline(cin, origional_string); }

string& StringModify::extract_word(string& a) {
    size_t position = a.find(" ");

    if(position != -1)
    {
        word = a.substr(0, position);
        a.erase (0, position + 1);
    }

    else
    {
        word = a;
        a = "";
    }

    return word; }

string& StringModify::reverse_word(string& s) {
    for(int i = 0; i < s.length() / 2; i++)
    {
        swap(s[i], s[s.length() - 1 - i]);
    }

    return s; }

void StringModify::rev() {
    string copy = origional_string;

    while (!copy.empty())
    {
        append(reverse_word(extract_word(copy)));
    } }

void StringModify::swap(char& v1, char& v2) {
    char temp = v1;
    v1 = v2;
    v2 = temp; }

void StringModify::append(const string& reverse_word) {
    formatted_string += reverse_word + " "; }

void StringModify::display() {
    cout << "\nThe original string: "
         << origional_string << "\n"
         << "\nThe formatted string: "
         << formatted_string << "\n"; }

Answer 1

您将结果分配a.find(" ")给a size_t. size_t是一个无符号类型; 它永远不会有负值.

请注意,这并不意味着比较永远不会成立.的-1将被转换为是无符号,以便能够进行所述比较(这是所谓的通常的算术转换部).转换-1为无符号时,它会产生无符号类型可表示的最大值.因此,如果find()返回最大可能size_t,那么比较将产生真.

要解决警告,您应该比较std::string::npos,find()如果找不到该元素,则返回的值.