为什么我的小程序在我运行后崩溃了

我想知道并理解为什么我的小控制台程序在我运行它后崩溃，就在一开始，即使它成功编译。

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
int main() {
    std::vector<std::string> valueWithUnit;
    {
        std::string unit = '';
        std::string convertNumb = '';
        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：

在 Project1.exe 中0x00191644时未处理的异常：0xC0000005：访问冲突读取位置0x00000000。 如果存在此异常的处理程序，则可以安全地继续该程序。

似乎问题与我的矢量有关，但我仍然不明白。提前谢谢。