c++ - 为什么我的小程序刚运行就崩溃了

Question

我想知道并理解为什么我的小控制台程序在我运行后就崩溃了，就在一开始，即使它编译成功。

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

int main() {
    std::vector<std::string> valueWithUnit;

    {
        std::string unit = '\0';
        std::string convertNumb = '\0';
        for (double forVal; std::cin >> forVal; ) {
            std::cin >> unit;

            if (unit != "cm" || unit != "m" || unit != "in" || unit != "ft") {
                std::cout << "The unit you entered is not supported by this program.";
                std::cout << " Try again with \"cm\", \"m\", \"in\", \"ft\"\n";
            }
            else {
                convertNumb = forVal;
                valueWithUnit.push_back(convertNumb + unit);

                if (valueWithUnit[valueWithUnit.size() - 1] == "100cm") {
                    std::cout << "That's also 1 meter.\n";
                }
                else if (valueWithUnit[valueWithUnit.size() - 1] == "2.54cm") {
                    std::cout << "That's also 1 inch.\n";
                }
                else if (valueWithUnit[valueWithUnit.size() - 1] == "1in") {
                    std::cout << "That's also 2.54 centimeters.\n";
                }
                else if (valueWithUnit[valueWithUnit.size() - 1] == "1ft") {
                    std::cout << "That's also 12 inches.\n";
                }
            }
        }
    }

    for (std::string i : valueWithUnit) {
        std::cout << i << std::endl;
    }

    system("pause");
    std::cin.ignore();
    std::cin.get();
    return 0;
}

当我调试它时，它告诉我:

Unhandled exception at 0x00191644 in Project1.exe: 0xC0000005: Access violation reading location 0x00000000. If there is a handler for this exception, the program may be safely continued.

问题似乎与我的 vector 有关，但我还是不明白。 提前致谢。

Answer 1

if (unit != "cm" || unit != "m" || unit != "in" || unit != "ft") {

如果此 if 表达式的计算结果为真，请注意执行路径不会将新值插入到 valueWithUnit vector 中。然后……

if (valueWithUnit[valueWithUnit.size() - 1] == "100cm") {

... 如果这是循环的初始、第一次迭代，则 valueWithUnit vector 仍为空，因为之前的 if 语句跳过了插入valueWithUnit vector 中的一个值。

因此，valueWithUnit.size() 将在此处返回零。您可以自己解决此错误的其余部分。